Talk:Greenwich Mean Time/Archive 1

UT1

UT1 is the same as GMT, isn't it? Currently UT1 redirects to Universal Time. —Ashley Y 13:40, 2005 Jan 24 (UTC)

Universal Time was invented as another name for GMT in 1928. UT was split into UT0, UT1, and UT2 in the 1960s. UTC was invented in 1972. But GMT has a much longer history than that. It began when the Royal Observatory was founded at Greenwich in 1675. For astronomers 00:00 GMT meant noon, but for civilians 00:00 GMT meant midnight, twelve hours earlier. As of 1 January 1925, the astronomical GMT day was shifted by twelve hours so that it also began at midnight. Thus GMT has not always been the same as UT1 by any stretch of the imagination. But UT1 has always been a type of Universal Time, thus the redirect is correct. — Joe Kress 08:02, Jan 25, 2005 (UTC)

You do not say whether astronomical GMT was changed at 1925-01-01 00:00:00 old GMT or at 1925-01-01 00:00:00 new GMT. Neither does the Article. IMHO, Wikipedia ahould record, perhaps here, that Royal Navy time was 12 hours AHEAD of local solar time, up to October 1805. Naval astronomers would have been a date behind their Captains. 82.163.24.100 (talk) 17:21, 27 September 2008 (UTC)

Astronomical GMT changed at 1925 January 1 00:00:00 new GMT. The old designation 1924 December 31 12:00:00 GMT which existed in the British Nautical Almanac and Astronomical Ephemeris for 1924 was relabeled in the 1925 edition as 1925 January 1 00:00:00 GMT, both meaning midnight. The American almanac refused to use the term "Greenwich Mean Time" in 1925 and later editions, feeling that the possibility of confusion was too great, so the American Ephemeris and Nautical Almanac for 1925 and later used the term "Greenwich Civil Time" for the new GMT, as did most other national ephemerides. To avoid confusion, it is now recommended that the old GMT for a day beginning at noon used in both almanacs be called "Greenwich Mean Astronomical Time". See page 76 of the Explanatory supplement to the Astronomical Almanac (1992). Although the International Astronomical Union directed that "Universal Time" replace both GCT and GMT in 1928, the ephemerides did not make any change. The American Ephemeris finally adopted the term "Universal Time" for its 1952 edition. The British Nautical Almanac and Astronomical Ephemeris adopted the dual term "Universal Time (UT) or Greenwich Mean Time (GMT)" in 1953. In 1960 the British finally dropped the term "Greenwich Mean Time", leaving only "Universal Time" in The Astronomical Ephemeris (the name Nautical Almanac was dropped in that year). However, GMT continued to be used in both the American and British Nautical Almanacs which were separately issued for seamen. See Mean solar time on the meridian of Greenwich. — Joe Kress (talk) 05:18, 29 September 2008 (UTC)

Dublin-Mean Time

Could Dublin-Mean Time be included in this section or on a seperate page.It was 25 minutes behind of Greenwich according to The Irish Times Book Of The Century.I can't find my copy of the book but anyone with a copy could add the page if it is considered notable.--Fenian Swine 22:59, 22 August 2005 (UTC)

Hmm lots of cities had local times. Why add Dublin in particlular? Theresa Knott (a tenth stroke) 11:56, 25 August 2005 (UTC)

Is that apart from the fact that Dublin is the greatest city in the World.I was unaware of other local times.Why not add a page with the other local times.--Muc Fíníneach 15:01, 25 August 2005 (UTC)

You are free to create a page entitled "Dublin mean time" if you can make it larger than a stub (stubs serve no useful purpose). See Washington mean time and Washington meridian. — Joe Kress 17:08, August 25, 2005 (UTC)

Zulu

The article stated: In the military, GMT is often referred to as "Zulu" time, ie 2000 Zulu would be 8:00pm in Greenwich. I have removed this. First of all, Zulu is another name for UTC, not GMT. Second of all, even if it were identical to GMT, then 2000 Zulu would 8:00pm in Greenwich only during winter, not during summer. --Netvor » user | talk | mail | work » 13:25, 15 March 2006 (UTC)

I think you're misunderstanding the meaning of GMT. Greenwich doesn't use GMT during the summer. GMT is an invariant time zone; the UK is on GMT for half the year and BST for the other half. 03:33, 20 September 2007 (UTC)03:33, 20 September 2007 (UTC) —Preceding unsigned comment added by 86.137.136.182 (talk)

Colored map slightly unclear

It is currently captioned: Dark colours indicate countries observing daylight saving. It's not clear which colors are dark. Are Russia and Eastern Europe "dark"? --Ds13 00:01, 5 May 2006 (UTC)

Yeah I noticed this too. I'm glad I'm not the only one. —Preceding unsigned comment added by 71.13.85.116 (talk) 03:38, 6 April 2008 (UTC)

211's addition

211, you previously agreed at Talk:Greenwich Mean Time/Time Cube not to re-add this material. This behavior is inappropriate. You may not add any such material without first convincing the other editors on the talk page. — Knowledge Seeker দ 08:14, 30 September 2006 (UTC)

Merging?

Why don't we merge UTC, UT1 and GMT together in to 1 article named maybe, International Time, etc. --Jacklau96 01:11, 19 November 2006 (UTC)

Because they are different things? GMT has had a longer history than e.g. UTC. Also UTC strictly is not the same as GMT. In any case, International Time (without a qualifier such as "Atomic") is a non-existent: we have Universal Time instead. Tom Peters 14:11, 19 November 2006 (UTC)

Does anyone still use real GMT?

UTC is the international standard and it seems that even the broadcast time signal in the UK is UTC. The article does not make it clear whether there is someone still keeping a clock set at real GMT and whether anyone uses it and why. OK now re-read. GMT and everyone's time zones based on UT1 and there is a shift every so often by leap seconds to UTC & UT in step. JMcC 19:03, 27 November 2006 (UTC)

ZULU = GMT = UTC Greenwich Mean Time = Co-ordinated Universal Time (UTC) = "ZULU" a short form for the aforementioned. Originally referred to by all radio operators as Greenwich Mean Time or GMT... the term was "universalized" in the 70/80's to UTC, Co-ordinated Univeral Time. The term "ZULU" is the equivilant of the aforementioned, when referring to GMT or UTC.. i.e. "1436 ZULU" would be 14:36 UTC or 14:36 GMT; the purpose of which was and is simply to have a single, universal time standard by which people may set schedules worldwide... i.e. to someone in North America, 1436 ZULU is the same as to someone in Africa, or anywhere else in the world. Makes setting schedules amongst, for example, radio amateurs (shortware radio operators) easier. The use of "ZULU" was originally adopted by the military and, as previously indicated, was the equivilant of GMT (without DST applied). "ZUKU" was used instead of GMT because it was a word that is easily understood over less than ideal communications links. See WWV, WWVH time standards.

UTC is not offically recognised or even heard of by the vast majority of the britsih public so throughout most of Europe it is commonly assumed that GMT is used rather than UTC. UTC is percieved publically as just anothe USA invention to be awkward.--Lucy-marie 16:35, 26 May 2007 (UTC)

Well, not quite. UTC is officially recognised in the UK, but not used by the British public - clearly not the same things. This is why the BBC World Service gives the time in GMT and the Shipping Forecasts on Radio 4 also state "Issued at 23:15 GMT". The same is true for some other measurements (e.g., the international standard unit for measuring atmospheric pressure is the hectopascal, but in UK aviation the millibar is still quoted. They are, in fact, identical!) Emeraude 11:20, 27 May 2007 (UTC)

Map

Somebody needs to add the Serbian–Montenegrin border to the map. Tomasboij 10:56, 2 August 2007 (UTC)

I'm copying your complaint to Template talk:Time zones of Europe. — Joe Kress 01:54, 4 August 2007 (UTC)

Loftus

I'm moving the following statement (added to the article by an anonymous editor) to its talk page because its claim is tenuous at best.

GMT also changed many place names as often there were two of the same name, an example of this is there were two Lofthouse's so one had to be renamed Loftus to avoid confuson of travellers.

The name was indeed changed during the nineteenth century from Lofthouse to Loftus as implied by the history of Loftus. However, it had been known as North and South Loftus during that seventeenth century according to the same history. But the name change must have occurred after 1859 because Lofthouse appears on a paten given to the Church of St. Leonhard in 1859, eleven years after all British railways adopted GMT in 1848. — Joe Kress (talk) 21:14, 31 December 2007 (UTC)

And even if the claim were true, it would need some serious sourcing to explain what such name confusion could possibly have to do with time zones. –Henning Makholm 01:26, 1 January 2008 (UTC)

anomaly in time in Greenwich, UK?

The article/map seems to be telling me that the time in Greenwich, UK in the summer is not Greenwich Mean Time. Details:

1. The article's top-right time-zone map seems to be saying that the time in Greenwich in the summer is UTC plus one hour, because such town seems to be located within London, UK.

2. The article states that "UTC" is in common usage the same as "Greenwich Mean Time", i.e. "Greenwich Mean Time (GMT) is ... now often used to refer to Coordinated Universal Time (UTC) ... ."

3. So, restating "1" above, the time in Greenwich in the summer is GMT plus one hour??

I must be missing some idea. Sorry in advance if there's some relevant clue given in an existing Talk section. Thanks in advance for suggested answers or keys to the puzzle.

Bo99 (talk) 16:50, 23 February 2008 (UTC)

You are not missing anything. The time in Greenwich during the summer is not Greenwich Mean Time. British Summer Time is one hour ahead of Greenwich Mean Time during the summer. Greenwich Mean Time is the time in Greenwich only during the winter. GMT does not experience one hour shifts. The mean in Greenwich Mean Time indicates that it is the average time by the Sun at the Royal Observatory, Greenwich. — Joe Kress (talk) 21:08, 23 February 2008 (UTC)

Thanks very much. I sort of kind of understand, i think.

The fact that the time in Greenwich during the summer is not Greenwich Mean Time seems like an anomaly, or something not obvious to readers trying to use the article, or something useful to such readers. Do you or others think that such fact could go in the article?, perhaps at the start of the Anomaly section (before the political anomalies), or elsewhere in the article. If so, i could try to do the writing, though i know you would be a surer hand at it.

Bo99 (talk) 01:11, 24 February 2008 (UTC)

I think it worked out as it did for historical reasons. Mariners had come to rely on charts that marked longitude from Greenwich, and set their chronometers to Greenwich, long before summer time was thought of. These mariners had no interest in whether or not the British Isles observed summer time; most of them were elsewhere. So they ignored the adoption of summer time in the British Isles. --Gerry Ashton (talk) 02:13, 24 February 2008 (UTC)

I've clarified it at the end of the introductory paragraph. — Joe Kress (talk) 02:29, 24 February 2008 (UTC)

Thanks again. Bo99 (talk) 13:43, 24 February 2008 (UTC)

"In the community of Greenwich, GMT (in the form of UTC) is the official time only during winter (during summer the time in Greenwich is British Summer Time rather than GMT)."

This information is irrelevant in the context of the topic, and misleading inferring it's just local to the community of Greenwich, rather than the entire UK.
Please remove it entirely or contextualize correctly into the 'Time Zone' section of the page. 86.135.51.232 (talk) 15:16, 30 May 2008 (UTC)

The Change of 1925

The article now says "The old astronomical convention (before 1925) was to refer to noon as zero hours, whereas the civil convention during the same period was to refer to midnight as zero hours.". But did the change occur at the end of London Civil 1924-12-31 or 12 hours later at the end of Astronomical 1924-12-31 ? 82.163.24.100 (talk) 21:17, 6 April 2008 (UTC)

The former, at the end of civil GMT 1924-12-31 midnight, hence astronmical 1924-12-31 was only 12 hours long, beginning at noon and ending the following midnight, according to all astronomical almanacs of the year, such as the 1925 American Ephemeris and Nautical Almanac. — Joe Kress (talk) 03:09, 7 April 2008 (UTC)

Prime meridian

"The international prime meridian is no longer precisely the Greenwich meridian, but remains close to it (5.31"E)."

The above statement is incorrect, and is referring to the WGS/ITRF zero meridian derived from NAD27, (as used currently by GPS). If disputed please provide source.

The international prime meridian was set at the International Meridian Conference in Washington 1884, adopting the meridian passing through the centre of the transit instrument at the Observatory of Greenwich as the initial meridian for longitude.

Also the 5.31"E is a potentially misleading format, please use arcseconds or a full d.mm.ss format. 86.135.51.232 (talk) 15:10, 30 May 2008 (UTC)

I replaced the statement challenged by 86.135.51.232 with a quote from a reliable source. --Gerry Ashton (talk) 17:09, 30 May 2008 (UTC)

I left the spelling of center unchanged. Check Howse to see if he used the British spelling centre. — Joe Kress (talk) 19:13, 30 May 2008 (UTC)

How to specify a clock time valid year-round?

There was a question posed above [1] that never got answered, but it should be addressed in the article. In North America, for example, a company might say “Our customer service hotline is available from 6am to 11pm PT.” The “PT” stands for Pacific Time, i.e. PST when standard time is in effect and PDT when daylight saving time is in effect. Is there a corresponding abbreviation that can be used in Great Britain? --Mathew5000 (talk) 10:59, 7 August 2008 (UTC)

Since GB, which can be extended to the UK or even to the British Isles, has the same time everywhere, there's no need for such an abbreviation here. LCT (Local Civil Time) can be used for that purpose anywhere, but needs explanation in each document. 82.163.24.100 (talk) 17:37, 27 September 2008 (UTC)

Merger (or something) needed

This article and Western European Time are talking about topics which are at least very closely related. Is there a actually a difference between the two? I note in particular the image on this page refers to Western European Time rather than GMT. Merging them (with an explanation of any differences between the two) would seem to be the best solution to me, but I'm open to more informed opinions. Cheers --Pak21 (talk) 10:06, 10 September 2008 (UTC)

GMT, albeit technically obsolete, is a term widely understood throughout the world as referring not only to the time in Great Britain from mid-c19 to 1916, and in winters (and some summers) thereafter, but also to a world standard time. It is part of the culture of, at least, the Commonwealth. It is an acronym like NATO, which has almost become a word. But WET and WEST have not, at least not in English.

The Article had only a single WET, explained only by context. Fixed.

82.163.24.100 (talk) 18:02, 27 September 2008 (UTC)

You're failing to look at the image caption! While a (unreferenced) comment has been added that GMT is "substantially equivalent" to Western European Time, there's still no explanation of what the difference (if any) actually is, or which source is being referred to here. --Pak21 (talk) 11:23, 6 October 2008 (UTC)

Whatever people actually use now - GMT has a historical significance that means there should be an article with this title. GSAckerman (talk) 06:32, 22 September 2008 (UTC)

Greenwich Mean Time was a standard used as the basis for the international system of time zones. The current standard is still based on Greenwich Mean Time, though there have been tiny adjustments to the definition, and the name of the standard is no longer "Greenwich Mean Time". Western European Time, on the other hand, is a name given in Europe to a time standard used in one of the international time zones. The name "Western European Time" is in fact a synonym for "UTC". The two expressions refer to different concepts: a basis for defining the whole international time zone system, and one of the particular time zones. Furthermore they give actual clock times which are very similar, but not quite identical. Also the historical significance of GMT surely gives it sufficient importance to justify a page of its own. JamesBWatson (talk) 21:17, 3 October 2008 (UTC)

Have to agree with sentiment that GMT needs its own article, particularly on historical grounds. Also, GMT is now a legacy standard, while UTC is a current one, albeit a close derivative. Socrates2008 (Talk) 22:42, 3 October 2008 (UTC)

GMT is still the legal basis of time in the UK and Ireland. Whether it means UTC, UT1, or something else is not entirely clear. This has been discussed at Talk:UTC.

There are two questions here, should the articles be merged and which should be merged into which. Given that Western European Time gives no explanation of where the term originates from or who uses it, I'd say it is in a pretty tenuous information as an article. There is also some useful information there that should be in GMT. -- Fursday 22:34, 14 October 2008 (UTC)

I'd certainly agree with all that. In the meantime, the merger tag has been removed by an anonymous IP; would anyone object if I readded it (possibly as a mergefrom/mergeto pair), as these are still, as far as anyone has been able to explain, two articles about exactly the same thing? Cheers --Pak21 (talk) 18:48, 16 October 2008 (UTC)

WET = UTC but ≠ GMT ninety:one 22:30, 26 October 2008 (UTC)

UTC suggests something different, implying that GMT is an ambiguous term that could be used to mean both the original GMT time system, UTC itself or UT1 which is closer to the original GMT. This article collaborates this view in its lead by saying "[GMT is] a term originally referring to mean solar time at the Royal Observatory in Greenwich, London [but] is now often used to refer to Coordinated Universal Time (UTC) when this is viewed as a time zone".

The scope of the GMT article should therefore be twofold, first to describe the historical purpose of GMT as the precursor to firstly UT and latterly UTC; second to describe the time zone based on UTC which it has come to mean.

Perhaps, even, there is a case for two articles; one called "Greenwich Mean Time" - describing the antiquated system in its precise and technical sense, and another called "Greenwich Mean Time (time zone)", the latter principally describing the UTC+0, to which Western European Time would redirect. That is assuming, of course, that Greenwich Mean Time is the most commonly used English Language term for UTC+0.

-- Fursday 03:31, 27 October 2008 (UTC)

Take a look at "National Legal Requirements for Coordinating with Universal Time". One of the authors, Seidelmann, is editor of one of the more important books dealing with time, The Explanatory Supplement to the Astronomical Almanac. The UK defines its legal time as GMT. However, no national time dissemenation service provides a time scale named GMT. It could be argued that UTC is an adequate approximation to GMT for legal use; I guess we'll never know until someone brings a lawsuit over the issue and it has to be settled by one of the upper-level courts of appeals. --Gerry Ashton (talk) 03:59, 27 October 2008 (UTC)

ninety:one - what's your source for that information? Cheers --Pak21 (talk) 11:07, 27 October 2008 (UTC)

UTC

I'd like to know when the UTC took over from GMT because I have never seen UTC before coming to this site, hower GMT is still widly used all over The Net and in Newspapers and Television.

I don't know. I believe that these days civil time in the UK is UTC and in summer UTC+1, and we call them GMT and BST. Certainly the pips are synched to UTC. Morwen - Talk 19:27, 22 Jun 2004 (UTC)

Within the UK (and, I presume, UK-registered/based shipping?) GMT and BST are the only names used for time as measured in the UK. People outside the UK however, not wanting to appear UK-centric, seem to utilise the alternative expression of 'UTC'. They are, for most practical purposes, the same (I believe the leap-seconds and other adjustments may not always be exactly equivalent, but for most practical purposes they are). --VampWillow 22:54, 22 Jun 2004 (UTC)

If you have a look into the subject of shipping in the United Kingdom (instead of making presumptions), you’ll see that, in this field, like in most of the world, the use of UTC, and not GMT or BST, is the working standard.

The United Kingdom Hydrographic Office’s Easy Tide product, which claims to be ‘the most comprehensive tidal prediction service on the Web’, gives predictions in ‘standard local time’ expressed in relation to UTC, not GMT.

Should you need to communicate with British maritime officials, you’ll need to date and time your communications in UTC, not GMT, as instructed in this document entitled Safety of British Merchant Ships in Periods of Peace, Tension, Crisis or Conflict issued by the UK Hydrographic Office and Ministry of Defence (Navy). The same practice is prescribed in Marine Guidance Notes like this one issued by the UK Maritime and Coast Guard Agency.

If you should be so unfortunate to have an accident while in UK waters, you’ll need to fill in an Incident Report Form from the UK Marine Accident Investigation Branch which asks that the time of the accident to be reported in ‘UTC or Local time’, not GMT. In fact, a search of the UK Marine Accident Investigation Branch website turns up 560 uses of UTC and only 32 uses of GMT. Usage of UTC is likewise overwhelmingly lopsided at the UK Air Accidents Investigation Branch where UTC is used 3974 times and GMT is used only 22 times. In fact, this corrigendum shows that British Summer Time was ‘incorrectly’ used instead of UTC on an air accident report.

Should you operate a fishing boat longer than 15 meters in UK waters, this document issued by the UK Marine and Fisheries Agency makes clear that your craft will need a satellite-tracking device that reports the date and time in UTC, not GMT.

A quick search of the The UK Met Office’s pages shows that, as of today’s date at least, the abbreviation UTC is used more often than GMT. The Met’s online version of the well-known Shipping Forecast avoids the use of UTC, GMT or BST altogether, leaving the user guessing what time scale is in use, but the detailed regional shipping forecasts issued by the Met Office, and broadcast via NAVTEX transmitters from Niton, Cullercoats and Portpatrick, are all time stamped UTC, not GMT. The same is true of the Met Office’s extended outlook and gales and storms warnings.

HM Coastguard broadcasts Maritime Safety Information, clearly marking the times of the broadcasts UTC, not GMT.

As these many examples make clear, in direct contradiction to your presumption, the use of the terms GMT and BST are not the only time scale designations used in UK shipping; in fact, the use of UTC is clearly as much used, if not more used, than either GMT or BST. --Blake the bookbinder (talk) 16:41, 15 June 2008 (UTC)

What in the UK should you use to denote that the time you are specifying is local in the sense that something occurs at 10AM - 10AM GMT in the winter and 10AM BST in the summer? Clearly if I specify just 10AM then it could be just about any time. If I say 10AM GMT then this will be the wrong time all summer. --KayEss 06:04, 14 Sep 2004 (UTC)

Wiki is the most amazing thing in the world -so useful- so informative-except when anything relating to the British comes up-whether in terms of identity or in this case time- time! After the person above wrote--'GMT is widely used all over the net and in newsapers and television'--

the editor answered (unbelievably!) 'I dont know-I believe that these days civil time in the UK is UTC and in summer UTC+1 and we (who is we?) call them GMT and BST---????? Is this person British????

The term UTC is NEVER used in Britain and for that matter-as the previous writer said -any where in the normal world where people are involved! The reason is simply that UTC is meaningless and confusing.It really means the time at Greenwich plus or minus a few hours-so why not stick to calling it Greenwich mean time?,-which is what of course most people in the world do because they can visualise an actual place on a map and see where they are in the world

This whole nonsense is only because the arrogant Americans dont want to have anything to do with the British!!But be careful America! In twentyfive years we all may have to change to Beijing time!89.132.129.209 (talk) 23:44, 25 December 2008 (UTC)

Have to say I disagree. Far more people use GMT in the UK than use UTC, the BBC world service gives its times in GMT, as do many British websites. You guys might not like it, but time in GMT is not going to go away any time soon. —Preceding unsigned comment added by FOARP (talk • contribs) 23:32, 10 February 2009 (UTC)

Amen. As an American, I have no problem at all with GMT. I do have a problem with UTC, which is that it purports to be a neutral term but actually conveys LESS INFORMATION because when you really get right down to it ... it's Greenwich anyway!!! So I say Good for Greenwich. They took the lead at the observatory there to standardize time, and so they get the credit for it. It's the same problem of FAKE PRECISION that we see in the term 'CE' or 'BCE' which in some contexts appear to have replaced 'AD' and 'BC'. It's the 'common era' BECAUSE it's before or after the birth of Christ (leaving aside the fact that this was miscalculated), so if you say 'BCE' instead of 'BC' you have just conveyed less information and attempted to hide the fact that it all refers to the birth of Christ anyway. Like the Greenwich observatory, the Church took the lead in standardizing years, so Jesus gets the credit for it instead of, say, Mahomet. Good for him.83.5.167.4 (talk) 09:41, 3 July 2009 (UTC)

UTC conveys more information than GMT because UTC, in practical terms, means the timescale broadcast by national time radio stations such as WWV and CHU. GMT means the mean time at Greenwich, but does not specify whether UT1 or UTC, which leaves an uncertainty of 0.9 second. --Jc3s5h (talk) 11:49, 3 July 2009 (UTC)

[From Terry0051] Sourced information about GMT/UTC questions: With respect to earlier comments, I suggest that the position is that 'UTC' does not convey either more or less information than 'GMT', because it is a different time scale. But the difference between the time scales is so small (less than 0.9 second) that for many purposes, when UTC is (inaccurately) called GMT, the small difference does not usually matter, just as it also does not usually matter for practical purposes that broadcast time signals are usually in UTC, while legal (winter) time in the nominally zero-offset time zone (in the UK) is actually still GMT and not UTC.

-- Also, this difference of up to 0.9 second between UTC and GMT (more precisely, UT1) is not an uncertainty: first because it is a difference between timescales and not an error; and second, the amount of the difference is regularly measured and kept track of, and continually made publicly available, to two distinct levels of precision. (UT1, in turn, is a refined or equalized version of the original UT or GMT, which would be the directly measured mean time at Greenwich, UT0 in present-day designations. The refinement is that UT1 includes equalization for the small effects of polar wandering on the longitude of the observing site. A formula relating UT1 and UT0 is given and sourced below.)

-- At the coarser level of precision for the difference between UTC and GMT/UT1, values are publicly available for the difference DUT1 = (UT1 - UTC) to the nearest 0.1 second, nowadays in scientific terms a somewhat coarse level of accuracy. This is often encoded in broadcast UTC time signals, but not as often used.

-- At the finer level of precision, a more accurate tracking of the difference (UT1-UTC) is constantly made and maintained; and this is publicly available both as predictions, and as retrospective final measurements. These can be found in bulletins of earth-orientation information available from IERS.

-- Also, the most recent edition (as of today 5 July) of the 'current' bulletin contains:

-- information about recent and future values of UT1-UTC, both in recent retrospect as measured values, and as predicted values for the coming 12 months, plus

-- information about the current broadcast value of the broadcast value of difference UT1-UTC mentioned above to the nearest 0.1 second.

-- For example, today 5 July, the coarse-precision value for DUT1 broadcast with the time-signals is +0.2 seconds, while the more precise (but predicted) value for UT1-UTC as at 0h today (and I think this means UTC here) was +0.23410 seconds. That will be superseded in due course by a measured value which probably will not differ by very much. (As an indication of the level of these differences, the last predicted value for 26 June was +0.23581 seconds, while the current measured value for the same date ('rapid service') is +.236015 seconds. That is only about a quarter of a millisecond different from the prediction. The 'rapid service' means an early report of measured values, and they are subject to usually only very small revisions to give final reviewed values later.)

-- Also contained in the same bulletins is information about the current parameters x and y for the state of polar wandering, and from these the difference UT0-UT1 can be obtained (according to info on p.50 in 'Explanatory Supplement to the Astronomical Almanac', 1992) as ( tan(lat)*( x*sin(long) + y*cos(long) ) ) (after conversion from angle to time). Terry0051 (talk) 15:24, 5 July 2009 (UTC)

The term GMT is used in a variety of ways, so unless the person using it states which meaning is intended, there is an uncertainty of close to 0.9 s not because there is that much uncertainty in the difference between UTC and UT1, but rather because there is that much uncertainty in what "GMT" means in any particular context. --Jc3s5h (talk) 15:51, 5 July 2009 (UTC)

{From Terry0051] Yes, with your specific qualification that it is an uncertainty of the usage of terminology and not of the time-scale, I quite agree with you (Jc3s5h). It's just that your specific qualification is often left out, leaving room for the statements to be meant and/or understood (incorrectly) in the sense that it is an intrinsic time-scale uncertainty in UT1 or GMT. Terry0051 (talk) 16:19, 5 July 2009 (UTC)

Is there a Wikipedia equivelent to LetMeGoogleThatForYou.com? Surely if anyone wants to know what UTC is, all they have to do is type it in the search box??? Or see UTC. How difficult can it be to work out that terms like 'Greenwich Mean Time' and 'Mountain Time' are locality specific monikers: the UTC+x notation is understood globally without need to find out what terminology is used locally. --Red King (talk) 15:40, 29 July 2009 (UTC)

Can we remove the idiot bias on this (read: American bias). UTC seems to only be in occurrence because American simpletons are butt hurt about standard time not being relevant to them. Leave this UTC nonsense to them and get rid of it from the article —Preceding unsigned comment added by 217.44.76.32 (talk) 12:50, 29 March 2010 (UTC)

The M means Mean?

I always thought it was Greenwich Meridian Time.

Everything I've read has GMT meaning Greenwich Mean Time, but some authors may have used GMT to mean Greenwich Meridian Time, which means almost the same thing. "Mean" indicates that the equation of time has been applied to the solar time at the Greenwich meridian. — Joe Kress 22:35, September 1, 2005 (UTC)

I also think so. I think someone in the last years begin to say "Mean" ,but mean is wrong because is not a mean time, but a clear sharp time give from the Meridian. —Preceding unsigned comment added by 79.24.121.104 (talk) 13:28, 1 January 2009 (UTC)

"Greenwich mean time" has been used since at least 1880. See eg the Statutes (Definition of Time) Act, 1880. --Pak21 (talk) 15:02, 1 January 2009 (UTC)

Greenwich Mean Time has been used ever since the Royal Observatory, Greenwich was built in 1676. However, for its first century GMT was only available on Thomas Tompion clocks with 13-foot pendulums within the observatory itself. When John Harrison built his H4 marine chronometer in 1761, it became possible for GMT to be physically transported outside the observatory. However, only other astronomical observatories and ships at sea initially used this GMT. Next the railways on the island of Great Britain began to use GMT in 1847, still physically transporting it via chronometers. GMT was first sent via telegraph in 1852. By 1855, virtually all public clocks on the island of Great Britain were using GMT (the island of Ireland used Dublin mean time). However, it was also calculated from transits of the Sun (true solar time), using the equation of time, and stars (sidereal time), using a fixed ratio, across the meridian of Greenwich using a transit circle within the Royal Observatory. Indeed, this 'virtual' GMT was used to synchronize the Greenwich clocks to Earth's rotation. Thus GMT has been both a physical reality and calculated since 1676. — Joe Kress (talk) 00:35, 2 January 2009 (UTC)

okok... ty very much :) —Preceding unsigned comment added by 79.41.125.76 (talk) 15:30, 2 January 2009 (UTC)

It's called 'Mean' time because the word is used in the sense of the average solar time between the east and west coasts of the UK - see Arithmetical mean. —Preceding unsigned comment added by 213.40.254.33 (talk) 09:39, 6 June 2009 (UTC)

Although it is an arithmetic mean, it is not the mean between the east and west coasts. Greenwich is much closer to the east coast than it is to the west coast of the island of Great Britain, so it is certainly not a coastal mean. The island of Ireland, still within the United Kingdom during the nineteenth century, used Dublin Mean Time until 1916. Dublin is on the east coast of Ireland, so its "mean" certainly is not the average between its east and west coasts. GMT is the arithmetic mean of the times that the Sun transits the meridian of Greenwich throughout the year. These transit times (sundial time) can be as much 16.5 minutes before noon clock time (Sun running fast) to as much as 14 minutes after clock time (Sun running slow), according to the equation of time. — Joe Kress (talk) 16:29, 6 June 2009 (UTC)

Major metropolitan areas

This seems like a POV list of random cities - you could include or exclude virtually any reasonably-sized place. There should be a firm criteria for a list of this type, e.g. population >1m (or whatever), otherwise it won't meet Wikipedia quality standards. Pondle (talk) 17:14, 14 November 2008 (UTC)

The entire section should be removed. I agree it is only a list of random cities. Nor does it improve the article. If a population criteria is used, a charge of geographic bias is possible because African cities are less populated than those in Europe. Regardless of any criteria we decide, even if explicitly stated in the article, it will be ignored and other random cities will be added. — Joe Kress (talk) 23:42, 14 November 2008 (UTC)

OK, I went ahead and removed the section. It Is Me Here (talk) 17:48, 15 November 2008 (UTC)

Anomalies

I added a section on 27 November 2008, addressing countries in the UTC "geographical" area that however choose to use UTC+1. Spain is an interesting example of political motives leading to a country's decision to move away from using its corresponding geographic time.

Cheers, sargoii —Preceding unsigned comment added by 71.56.252.178 (talk) 23:38, 27 November 2008 (UTC)

Archiving

This talk page is getting long. Does anyone object to me setting up archiving using MiszaBot? An example of a talk page with that setup is Talk:Gregorian calendar. --Jc3s5h (talk) 00:24, 9 July 2009 (UTC)

[From Terry0051] Sounds sensible to me. Just to make sure the preceding discussions are not in practice forgotten I took the opportunity to incorporate a few amendments to clarify/remove a couple of ambiguities/correct anachronism about UT1. Terry0051 (talk) 11:19, 9 July 2009 (UTC)

Prime Meridian

The section on the prime meridian is no longer quite correct: when the GPS system was set up, a choice of baseline had to be made as many measurements had previously been slightly inaccurate. The decision was made to preserve the 90 degree line where it was and use that as a reference (coincidentally, this is the line that runs through the US ;) and hence the meridian at Greenwich is now about 100m of its traditional location (which is great for confusing tourists clutching GPS gear...) Calum (talk) 11:12, 5 November 2009 (UTC)

[From Terry0051] That's interesting -- is there any RS for a "decision .. to preserve the 90 degree line"? Terry0051 (talk) 12:05, 5 November 2009 (UTC)

I do not believe this statement. The fundamental reference system used by the Global Positioning System is WGS84. Zero longitude in WGS84 is the IERS Reference Meridian which is also the zero longitude of the International Terrestrial Reference System (an ideal system) and the International Terrestrial Reference Frame (a practical system). I suppose a lay person's description would be a position based on an average of radio telescopes that can measure extremely distant radio sources, the moon, or special-purpose satellites. Because the radio telescopes are located on different continents, the averaging process tends to cancel out continental drift. At the time the radio techniques came into use, I'm sure there was an effort to match the Prime Meridian that was determined optically with transit circles. I am not able to give a step-by-step analysis of all the measurement tolerances and compromises that lead to the present 100 m discrepancy.

The part about the IERS Reference Meridian being about 100 m east of Airy's transit circle is true. --Jc3s5h (talk) 17:30, 5 November 2009 (UTC)

The source of Calum's claim is NADCON, which shows no change in longitude between NAD27 and NAD83 (=WGS84) just east of 90°W, roughly along 87°W. Although most of this was intentional, not all of it was. "Navigation at the Prime Meridian" (1971), cited in IERS Reference Meridian, states "The original longitude reference of the APL [world] datum was selected as the surveyed longitude of the [Applied Physics] Laboratory's site in Howard County, Maryland, a coordinate in the North American datum [1927]." The Applied Physics Laboratory is located midway between Washington, DC and Baltimore with a longitude of 76°50'43"W (and a latitude of 39°9'58"N). It developed the TRANSIT satellite Doppler navigation system beginning in 1958. The calculated shift using a standard Molodensky transformation from the non geocentric NAD27 (the center of its ellipsoid is 241 m away from Earth's center) to the geocentric NAD83 [2] yields a large 8.34" or 161 m shift at Greenwich. But actual measurement shows that the APL reference meridian was 5.64" (108.8 m) east of Airy's transit circle in June 1969, long before the BIH adopted an earth centered ellipsoid in 1984. The MERIT/COTES campaigns of 1983–84 added lunar laser ranging, satellite laser ranging, and Very Long Baseline Interferometry. These resulted in a small shift to the west of 0.806" (15.6 m) from TRANSIT's reference meridian,[3] which roughly corresponds to the current 1.09" shift at APL according to NADCON. Currently, the IERS reference meridian is 5.31" (102.5 m) east of Airy's transit instrument.

The History of the BIH (§5) has a different reason for the 100 meter shift. It states that it was caused by the continuity condition on UT1, that is, that UT1 should not exhibit any discontinuity due to the changeover in 1984 from time (and longitude) based on the meridian transits of fixed stars at several astronomical observatories to time based on observations of satellites, both artificial (SLR and Doppler) and the Moon (LLR). Indeed, there was no sudden shift in UT1 in 1984.[4] Before 1984, the BIH calculated UT1 as an average of the time determined by several national astronomical observatories, corrected for propogation delay, but not corrected for the several datums (non geocentric ellipsoids) used by those observatories. I suspect that this continuity condition on UT1 determined the reference longitude for the SLR/LLR/VLBI frame, hence caused the final 15.6 m shift in 1984. If TRANSIT is ignored, the entire shift could be attributed to UT1.

The opinion of the Royal Observatory itself, now a tourist attraction, that the 100 meter shift was the result of the accretion of several small inaccuracies, could also be included in the pre-1984 history.[5] So every one of these reasons is correct in its own way. — Joe Kress (talk) 03:51, 16 November 2009 (UTC)

Not so much inaccuracies, but since the earth is not a perfect ellipsoid each country's scientists made the best fit approximation but did so to minimise errors in their own country. Since GPS is an American system it fits best there, its not really a coincidence, if it was English there would be no error. QuentinUK (talk) 13:55, 14 April 2011 (UTC)

I would not suppose that GPS fits better in the U.S. than elsewhere. After all, it was initiated by the U.S. Dept. of Defense, and they usually fight outside North America, so I would expect equal treatment of the whole world, or at least the land areas. Jc3s5h (talk) 21:31, 14 April 2011 (UTC)

GMT date format

Can't see info about GMT date format. I've a short note to that in my head, for example: GMT date format (time format) uses the ISO 8601 standard of YYYY-MM-DD HH:MM:SS GMT. Or maybe simple internal wiki reference to ISO_8601 would be enough. Hondrej (talk) 16:08, 25 October 2010 (UTC)

The article does not mention a date format because GMT does not have any. ISO 8601 requires the use of UTC and even states that UTC is not GMT. Nautical navigation, which does use GMT, specifies that its GMT is UT1, not UTC, where UT1 is the mean solar time at the Royal Observatory, not its atomic clock approximation, UTC, which can be in error by as much as 0.9 seconds. No date format is specified in Dutton's Nautical Navigation (2004), but it uses both day/month and month/day, where the month is either spelled out or uses a three letter abbreviation. — Joe Kress (talk) 00:21, 26 October 2010 (UTC)

Format

Is it just me or does this article look grossly out of format? I see stuff that looks like a Courier fonted DOS page on my screen for this article! Ratibgreat (talk) 17:16, 1 April 2011 (UTC)

It generally looks normal to me. Was there a specific passage you had in mind? If so, please quote a bit of it so we can search for it using our browser's search feature. Jc3s5h (talk) 19:55, 1 April 2011 (UTC)

No, apparently everything's back to normal. It looked like a DOS page, I tell you!Ratibgreat (talk) 07:37, 4 April 2011 (UTC)

I have seen some Wikipedia pages, usually talk pages while I'm previewing them after adding a comment but before saving it, with monospaced font and no Wikipedia skin whatsoever, over a week ago, although not within the last week. — Joe Kress (talk) 21:21, 4 April 2011 (UTC)

I've seen this happen. It looks like the Wikipedia server is getting confused about what type of computer is connected and formats the page for a portable device. QuentinUK (talk) 13:39, 14 April 2011 (UTC)

GMT is 'atomic time', or 'earth angle time' ?

[Note: above heading was added at 13:08, 2 October 2011 by User: CorvetteZ51.

There is no answer for this. The scientific community has retired the term "GMT". Some countries, like the UK, have laws that set civil time as GMT, or an offset from GMT, but the UK parliament has declined to make any law clarifying the current meaning of GMT in the law. As far as I know, no other country with such laws has clarified the meaning of GMT since the introduction of UTC. Jc3s5h (talk) 14:27, 2 October 2011 (UTC)

OK, I give up. UTC is what I call, 'cogged - international atomic time', which is kept close to the mean-sun with 'leap seconds'. UTC is always adjusted in units of a whole second. OTOH, UT-zero is continuously adjustable and represents the angle of the earth. which of those two us closer to GMT? CorvetteZ51 (talk) 09:47, 6 October 2011 (UTC)

The only country I know of that officially bases its time scale on "Greenwich Mean Time" is the UK. Who knows if their law is even valid, it might be overridden by some European Community law. Of course, there may be other countries that base their time on GMT, I just don't know about them.

The UK House of Lords debated a bill that would have changed to UTC. One sentence from the debate is "The use of Greenwich Mean Time for scientific purposes duly died out, although the term lives on in everyday parlance. One reason the scientific community no longer uses the term GMT--as the noble Lord, Lord Tanlaw, explained to us--is that we are no longer really sure what GMT is." The bill did not pass. So my interpretation is that the UK has been asked to clarify what GMT is, or else stop using it, and they refused to act. Jc3s5h (talk) 12:00, 6 October 2011 (UTC)

Of course we refused. Greenwich is in England, and we invented the concept of standard time. --Redrose64 (talk) 12:57, 6 October 2011 (UTC)

Links

I have added some more wiki links on "time zone", trying not to overdo it.

Is there a policy on multiple links to terms used frequently in an article?

My personal preference would be one & no more than one per para, or perhaps 1 per section; one per article is not enough IMHO, especially in a long article, where the reader may only be interested in one section. D A Patriarche (talk) 05:23, 3 March 2014 (UTC)

• From the MoS: "Generally, a link should appear only once in an article, but if helpful for readers, links may be repeated in infoboxes, tables, image captions, footnotes, and at the first occurrence after the lead." Bjelleklang - talk 08:00, 3 March 2014 (UTC)

House of Lords debate

In the UK the law says the time (in winter) is GMT, but the National Physical Laboratory time transmissions are UTC. A bill was introduced in the House of Lords to settle the confusion by declaring declaring that GMT is UTC, but the law was never passed by the House of Lords. The debate may be found here.

This is why we can't make statements like "GMT is the same as UTC" or "GMT is different from UTC". Some authorities think they're the same, some think they're different, and the House of Lords refuses to answer the question. Jc3s5h (talk) 22:10, 13 August 2014 (UTC)

Paris

Paris does not, as stated in the article lie East of 7 Degrees 30 Minutes, it's Longitude is in fact 2°20′14.025″ East. This means that local noon occurs just after 11:49 GMT or UCT. The reason why France does not keep GMT and this was recorded at the nference in 1884 which fixed the Greenwich Meridian as the Prime Meridan, is because France / Paris was not selected by the majority of delegates. Ever since then France has refused to accept GMT / UTC as the time reference.The Geologist (talk) 15:31, 18 January 2013 (UTC)

Although your history may seem reasonable given Anglo-French competitiveness, it is wrong. The French delgates to the International Meridian Conference never proposed that the Paris meridian should be Earth's Prime Meridian. Instead they proposed a neutral meridian that would not cut Europe or America, suggesting either the Azores or the Bering Strait. (Proceedings of the International Meridian Conference, pp. 36-84, vote pp. 84-85) The only other initial meridian proposed was Greenwich. [pp.87-99] Because most delegates did not adopt the French proposal, France abstained from voting on all proposals that mentioned or implied Greenwich. accepted resolutions and votes, pp. 199-203

Nevertheless, France kept Greenwich Mean Time for most of the 20th century, but without using the word "Greenwich" in the law. France had already adopted Paris Mean Time for the entire country on 14 March 1891 (except that all French railway times were five minutes slow). In March 1897 a bill was introduced into the Chamber of Deputies that stated "The legal time in France and Algeria is the mean time of Paris, retarded by 9 minutes, 21 seconds." This bill languished in the Senate for 12 and a half years. Beginning midnight March 10/11, 1911, France's legal time was defined as "Paris Mean Time, retarded by nine minutes twenty-one seconds", which happens to be Greenwich Mean Time. The bill also made French railway times conform. Despite the lack of "Greenwich" in the law, newspaper articles stated that this was really l'heure anglaise de Greenwich. Time signals transmitted from the Eiffel Tower changed to Greenwich mean time on 1 July 1911. French hydrographic charts adopted the Greenwich meridian as of 1 January 1914 as a direct result of the sinking of the Titanic on 15 April 1912. On 25 March 1917, zone time based on Greenwich was adopted by French ships at sea, which had been using apparent time. A detailed discussion is provided by Ian R. Bartky, One time fits all (2007) pp. 127-134, 138-153. This remained French legal time until 1978 when Coordinated Univrsal Time was adopted, which also does not use the word "Greenwich". [6][7][8] The equivalent astronomic difference in longitude (including vertical deflection) between Paris and Greenwich corresponding to 9 minutes 21 seconds was 2°20'15". The longitude of the Paris Observatory (Cassini's meridian) was given in the Astronomical Almanac through 1980 as -9m20.91s (-2°20'13.65"), and from 1981 to the present (2013) as -2°20.2' (-2°20'12" or -9m20.8s). — Joe Kress (talk) 00:58, 12 February 2013 (UTC)

At the time the vast majority of the World's shipping was British-registered and as the Royal Navy under such people as George Vancouver, James Cook, William Bligh, et-al, had made accurate Admiralty charts of places that no-one else had ever surveyed (hence if the French, Russians or anyone else wanted safe travel for their ships in these areas they had to use the only available accurate navigational charts, i.e, Admiralty ones) the logical choice for any international primary meridian was Greenwich and therefore GMT, as that was what the RN had used in all its charts, and it was their charts - or illicit copies of them - that were used by almost the entire world shipping community, both civilian and naval.

That's why Greenwich was chosen rather than Paris. — Preceding unsigned comment added by 95.148.220.15 (talk) 17:22, 6 February 2015 (UTC)

GMT is not UTC

The opening paragraph says that GMT "is arguably the same as Coordinated Universal Time (UTC)." This statement should be removed or changed to "GMT is a time zone subject to daylight saving time rules; UTC is not a time zone. During summer hours, they differ by one hour."

I've noticed a lot of websites saying that GMT and UTC are the same. They differ in every respect, including value (during summer months). There is no argument; they are not the same.

BTW, it was called Greenwich Meridian Time when I was a kid. When did it change to Greenwich Mean Time? Or was I taught the wrong thing way back then.

Warren Gaebel, B.A., B.C.S. 70.27.25.246 (talk) 17:11, 22 November 2012 (UTC)

You were taught incorrectly; it has always been Greenwich Mean Time. This makes the important distinction between mean solar time and true solar time (the time kept by a sundial).

— The Geologist — (continues after insertion below)

It always was Greenwich Mean Time and always has been, but a lot of modern teachers have decided that the way things were done was wrong therefore their way is right. Oh and I went to school in the 1950's and 60's. Even my grandfather who died in 1957 always referred to time as Greenwich Mean Time.The Geologist (talk) 15:36, 18 January 2013 (UTC), PhD, FGS.

I would suggest that "Greenwich Mean Time" is undefined, except in certain limited contexts. I defy you to find a definition from an authority over a large part of the world that gives a current definition of "Greenwich Mean Time" that can be unambiguously determined to a small fraction of a second.

I do think that paragraph needs to be cleaned up to explain that GMT is sometimes used as a synonym for UTC (especially outside the British Isles, where people are uninterested in British civil time), but is also used as a synonym for British civil time. Appropriate citations should be given for both meanings. Jc3s5h (talk) 17:57, 22 November 2012 (UTC)

I too remember reading "Greenwich Meridian Time" at times. Google Books clearly supports this.

I'm here because the Coordinated Universal Time, Greenwich Mean Time, Universal Time, and Leap second articles are confusing as to if and how UTC and GMT stay in sync. When leap seconds are added/removed from UTC does this also affect GMT?

Jc3s5h, I agreed with your comment that GMT is is undefined, except in certain limited contexts but we may need to have that stated in a reliable secondary source. The article should also show the various organizations that define GMT and include exactly how they define GMT and where that definition is legally applicable. --Marc Kupper|talk 19:37, 1 December 2012 (UTC)

I think the most appropriate source is the most recent, 3rd, edition of the Explanatory Supplement to the Astronomical Almanac. The contributing authors come from organizations such as the US Naval Observatory, US Geological Survey, Her Majesty's Nautical Almanac Office, National Geodetic Survey, and the Jet Propulsion Laboratory. At least one of these, the US Naval Observatory, is responsible (together with the National Institute of Standards and Technology) for dissemination of legal time in the US. On page 231–2 that source states:

In the United Kingdom, Greenwich Mean Time has been identified with the civil time or Coordinated Universal Time, UTC (§ 6.8.3.1). This connection, however, has never been formalized, so using GMT to refer to UTC should be done with care. For navigation, however, Greenwich Mean Time has meant UT1 (§ 6.8.3). Thus, GMT has two meanings that can differ by as much as 0.9 s, and the term GMT should not be used for precise purposes.

Of course this passage does not directly address to describing British summer time as GMT.

To answer the question about leap seconds and GMT, when GMT is interpreted as UTC, the leap seconds are inserted into UTC and GMT is just another name for this timescale. When GMT is interpreted as UT1, the seconds of UT1, defined by the rotation of the Earth, are slightly different from, and generally longer than, the atomic seconds of UTC. When the accumulated difference approaches 0.9 s a leap second is inserted into UTC to keep it close to UT1. Jc3s5h (talk) 20:06, 1 December 2012 (UTC)

Thank you. Do you know what "§6.8.3.1" and "§6.8.3" are referring to? I'm guessing they are sections within Explanatory Supplement to the Astronomical Almanac itself but wanted to confirm that.

I looked at the Universal Time article to better understand UT1. Something that's not clear is if one second in UT1 is exactly one "International second" or if it's 1/86,400 of a mean solar day. --Marc Kupper|talk 04:22, 4 December 2012 (UTC)

Yes, "§6.8.3.1" and "§6.8.3" are indeed references to other sections in the same chapter of the Explanatory Supplement.

One second of UT1 is effectively one second of mean solar time, because UT1 is the only time scale provided by the scientific community that is intended to be close to mean solar time. However, the expression for UT1 is a linear function of the Earth Rotation Angle, which in turn uses Very-long-baseline interferometry to measure the rotation of the Earth with respect to distant pulsars in other galaxies. The actual position of the Sun is not used in the calculation of UT1. The coefficients in the equation for UT1 were chosen for continuity with the previous definition of sidereal time. If you trace the various definitions back far enough, it turns out that indirectly UT1 is based on the position of the Sun, but there is no direct statement about that in the current definition.

In a paper in a 2011 (v. 48) special issue of Metrologica devoted to time, on page S182, Bernard Guinot wrote

Strictly speaking, the Universal Time, version UT1, is

not a solar time. It is a parameter which, jointly with the coordinates of the moving pole of rotation of the Earth, describes the rotation of the surface of the Earth in space. The knowledge of these parameters is essential for all space techniques with multiple applications, both practical and

scientific, of the utmost importance.

However, all decisions taken until now preserved the role

of UT1, the representation of the mean solar time at the Greenwich meridian as defined by the 1884 Conference, with

a departure which may reach one to two seconds.

It should be noted that since navigational almanacs and software are designed to be used with UT1, the one or two seconds from actual mean solar time described by Guinot will not result in the kinds of small navigation errors that result if navigators don't take the difference between UTC and UT1 into account. Jc3s5h (talk) 09:59, 4 December 2012 (UTC)

"During summer hours, they differ by one hour" by 70.27.25.246 is wrong. GMT is always approximately equal to UTC, never differs by one hour. Because GMT is the time only used in winter in Britain, while BST is the time used in summer in Britain. GMT is approximately equal to UTC, while BST is approximately equal to UTC+1. (P.S. the term "approximately" here is only for "leap seconds") -- Yejianfei (talk) 16:46, 30 January 2015 (UTC)

With all due respect, you are just a Wikipedia editor. There is no particular reason for anyone to pay any attention to your pronouncements. Please look at Greenwich Mean Time#Summer time where reliable sources are cited which make contrary claims; one says GMT is always approximately UT, the other says sometimes the summer time observed in the UK is called GMT. Can you provide a really convincing reliable source that proves one of the cited sources is spouting nonsense? And I'm not talking official definitions, I'm talking about all strata of people in the UK from the National Physical Laboratory down to the sign giving the operating hours of a fishmonger. Jc3s5h (talk) 17:51, 30 January 2015 (UTC)

"Greenwich Meridian Time" is a canard same as "After Death" for the abbreviation "AD". Language is conservative. Thus we still "hang up" the telephone even though that action has not been performed since before the war. So because we used Greenwich mean time before 1972 we still call our winter time GMT although it's something completely different.

I can tell you from here in the UK that the population does not and never has referred to British Summer Time as "Greenwich mean time". I'm going to check these articles and if I find any such claims out they will go. 156.61.250.250 (talk) 09:52, 31 January 2015 (UTC)

User Jc3s5h wrote: "I do think that paragraph needs to be cleaned up to explain that GMT is sometimes used as a synonym for UTC (especially outside the British Isles, where people are uninterested in British civil time), but is also used as a synonym for British civil time." User Marc Kupper wrote: "I'm here because the Coordinated Universal Time, Greenwich Mean Time, Universal Time, and Leap second articles are confusing as to if and how UTC and GMT stay in sync. When leap seconds are added/removed from UTC does this also affect GMT?"

I just edited the introduction section thoroughly because I found it confusing and a bit misleading. I added a note that GMT's function as the international standard civil time has been superseded by UTC; that UTC is kept within 0.9 of UT1 by adding/subtracting leap seconds; and that "GMT" is still misused as a synonym for UTC in Britain and some other places. Where the time zones used in Britain are mentioned, I explain that "GMT" and "BST" actually mean UTC+0 and UTC+1.

I hope this is satisfactory. Teemu Leisti (talk) 23:38, 25 April 2015 (UTC)

Thank you for your help in sorting out the confusing usages. Dbfirs 06:42, 26 April 2015 (UTC)

No worries. Though what I saw as clarification -- that "GMT" is generally misused as a synonym for UTC+0 -- has now been edited out. I really don't have the time and energy at the moment to defend my opinion on what should appear here. Teemu Leisti (talk) 11:07, 27 April 2015 (UTC)

Dispute accuracy of History section

The claim in the "History" section, introduced by User:156.61.250.250‎, that GMT is defined as UT1, is not true.

As Bernard Guinot writes in the abstract of his paper, "The International Conference held in 1884 at Washington defined a universal time as the mean solar time at the Greenwich meridian (GMT). Now, the Universal Time, version UT1, is strictly defined as proportional to the angle of rotation of the Earth in space. In this evolution, the departure of UT1 from GMT does not exceed one or two seconds." Clearly there cannot be a departure between two time scales if they are defined to be the the same. Jc3s5h (talk) 20:07, 18 April 2015 (UTC)

You're citing the abstract of the paper, not the paper itself? Unless you remedy that I'll remove the tag. The only circumstance in which Guinot would be relevant would be if he had experimentally timed an event by both GMT and UTC1 and found a discrepancy. 156.61.250.250 (talk) 12:16, 19 April 2015 (UTC)

I read the paper. Guinot has been involved in many of the critical papers involved in time and earth orientation; anonymous Wikipedia editors have no standing to question his qualifications to make a statement (although other high-quality sources could be used to show other points of view or other terminology preferences).

Once such difference in terminology preference is with the online glossary of the Astronomical Almanac which says "In current usage, UT refers either to a time scale called UT1 or to Coordinated Universal Time (UTC); in this volume, UT always refers to UT1." Guinot does not limit the current meaning so narrowly:

The name of a time and of a time scale either indicates its goal (such as Universal Time) or the mode of construction (such as Ephemeris Time), or both (such as International Atomic Time). Universal Time is a nice name, although stamped with the grandiloquence of the 19th century, because it indicates the goal of its definition, not the technique to build the time scale. But the need to distinguish versions of UT adorned its acronym with suffixes: Universal Time has versions UT0, UT1, UT2, UTR, UTC. As shown by Sadler [9] this situation is far from being satisfactory. [p. S185]

The paper does not define Greenwich Mean Time in so many words, but begins with the methods stated or implied by the 1884 Washington Conference that selected the Airy Transit Circle at the Greenwich Observatory as 0° longitude, measurement of actual transits of the Sun rather than measuring star transits, and using the vertical indicated by spirit levels at Greenwich rather than the normal to a reference ellipsoid. He then describes various changes in techniques and definitions, and estimates how much departure each change would contribute to the difference between the methods implied by the 1884 conference and UT1. I understand the structure of the paper, combined with the abstract, to mean Guinot considered Greenwich Mean Time to be the time measured in the way indicated in the 1884 conference.

Guinot (p. 182) also mentions "In 1955, the International Astronomical Union (IAU) officially decided to select a fixed point on the moving equator as the origin of longitude and to call UT1 (Universal Time 1) the universal time referred to this point." McCarty and Seidelmann (p. 17) devote most of a page explaining why "the use of the term 'Greenwich Mean Time' or its abbreviation 'GMT' remains a source of confusion today." They point out several times when respected authorities have stopped using GMT or recommended that GMT should no longer be used: the IAU in 1928, the 1939 American Ephemeris, astronomical almanacs after 1960, and another recommendation from the IAU in 1976. Since the same organization, the IAU, recommended the discontinuance of GMT in 1928 and introduced UT1 in 1955, it seems implausible they would define GMT as UT1; I would want to see a citation to an IAU publication saying they did such a strange thing before I would believe it. Jc3s5h (talk) 13:06, 19 April 2015 (UTC)

Never mind what the IAU recommended - they are not a judicial body. The British Parliament legislated Greenwich Mean Time back in the 1880s. It has not repealed that legislation, so people like yourself who suggest that leap seconds apply to Greenwich Mean Time are wrong. You have not answered the question I posed - does Guinot have experimental evidence that Greenwich Mean Time has drifted from UT1 - yes or no? Are you suggesting that in all those years from the introduction of Universal Time in 1925 astronomers had no idea of the relationship between it and Greenwich Mean Time?

In Talk:Coordinated Universal Time I explained what the Royal Greenwich Observatory has to say on the matter:

UT universal time; counted from 0h at midnight; unit is mean solar day

prior to 1925 [Greenwich Mean Time] was reckoned, for astronomical purposes, from Greenwich mean noon.

86,400 seconds of Greenwich Mean Time is one mean solar day and it's counted from 0h at midnight: thus it is (and always has been) in frequency and phase exactly the same as Universal Time. The Royal Greenwich Observatory has decided that, unless otherwise stated, "Universal Time" means UT1. Therefore, Greenwich Mean Time is UT1.

The idea that our legislators cannot tell the time is only in your head. The 1968 Standard Time Act provided that the legal time throughout the year should be Greenwich Mean Time plus one hour. If they did not know what Greenwich Mean Time was they would not have said that. They also said that "Nothing in this statute shall affect the use of Greenwich Mean Time in astronomy and navigation". Astronomers and navigators know exactly what Greenwich Mean Time is - if they didn't there would be an awful lot of shipwrecks which hasn't happened. 156.61.250.250 (talk) 15:04, 19 April 2015 (UTC)

If 156.61.250.250 has questions about Guinot's paper he or she should read it. As for the British legislation on time, this is discussed in a paper by Seago, Seidelmann, and Allen. UTC is the defacto legal time throughout the world, and the UK government funds the National Physical Laboratory, which disseminates UTC to the British public, and who then conduct their affairs according to that time scale, while usually describing it as GMT. As for what the Royal Greenwich Observatory says, there are several organizations that have existed with variations of that name, so I can't comment on what they say without seeing the exact text. Jc3s5h (talk) 16:32, 19 April 2015 (UTC)

Concerning the comment above that "If they [legislators] did not know what Greenwich Mean Time was they would not have said that" one might find it entertaining to read relevant debates in the House of Lords:

• 27 November 1996
• 11 June 1997

Jc3s5h (talk) 19:13, 19 April 2015 (UTC)

Lord Fraser of Carmyllie is (or was) a politician, not a scientist. However, he points out that Coordinated Universal Time and Greenwich Mean Time are in agreement to less than one second. Coordinated Universal Time is also in agreement with UT1 to within a second, so that throws Guinot's two second difference out of the window. Since his core argument has been disproved, why bother to read his paper?

You may recall that Lord Chesterfield, who piloted the Gregorian calendar through the Lords in 1750, admitted that he knew very little about the subject, but just made some superficially learned argument to stop the noble lords getting bored and throwing out the measure. Same here.

Greenwich Mean Time, which is not Greenwich Mean Time any more because it is not calculated on a mean - at noon.

is obvious nonsense.

This is an official statement by the British Government confirming that Greenwich Mean Time is not Coordinated Universal Time. Proof of Lord Fraser's ignorance is his conflating delta T and the equation of time. Be that at it may, as the official representative of Her Majesty's Government he confirms that Greenwich Mean Time is and will remain the legal time in these islands. You have said you are not legally trained. Your reference to a "de facto" standard being a legal standard confirms this. The two concepts are opposites.

Further down Lord Fraser categorically confirms that Greenwich Mean Time will always be within 0.9 seconds of Coordinated Universal Time. Since UT1 is kept within the same range GMT=UT1. The statement that the fictitious mean sun was abandoned in 1925 is wrong. It is still used to derive the analemma by which the relationship of sundial time to mean time is established.

I see from Lord Tanlaw's speech that ITU Recommendation TF460 of 1975 was that GMT should continue to be equivalent to Universal Time. Why didn't you mention that? What is the serial number and content of the 1976 recommendation which you do mention? The debate proceeded on the basis that GMT = UT1. The Bill was rejected and that is the end of the matter.

As for your claim that the British public use UTC and call it GMT that is rubbish. I used to work in Westminster, and every fifteen minutes we would hear the bongs of Big Ben. Sound travels at 1100 feet per second, so some people hearing those bongs might set their watches to GMT, others to UTC, others to UT0, others to UT2, others to UTR and so on. The precision of Big Ben is maintained by placing pennies on the mechanism to ensure the correct weight, so the claim that it can distinguish to sub - second levels of accuracy is risible. We are not as stupid as you think. A time display on a GPS unit will be treated as a GPS time, a UTC display will be treated as UTC, a computer display as that of the relevant operating system and so on. Nobody checking a digital watch is going to argue about whether it displays GMT or UTC. There is only one observatory at Greenwich, although the observing moved to Herstmonceux, but the civil service personnel involved are the same.

To wrap this, the RGO publishes times of astronomical phenomena, calculated according to UT1. These are reproduced in almanacs, calendars, diaries and newspapers which inform readers that these are GMT times or, (as the case may be), British Summer times, where the one - hour offset is applied. Apart from that, there is no discrepancy whatsoever between these times, so GMT = UT1.

There is one caveat - before 1925 publishers were sometimes caught out by the 12 - hour difference in starting the day, so sometimes their times are twelve hours out. 156.61.250.250 (talk) 11:20, 20 April 2015 (UTC)

There is no official conclusion from these debates, because the bill they were debating never became law. The general sense of the debate I get is the speakers thought there was a distinction between GMT and UTC, but they also thought that the time being disseminated to the general public, often under the name GMT (in winter), was actually UTC. There was clearly no consensus among the speakers that GMT was UT1; it could be UT0, UT1, UT2, or a hypothetical UT measured with the Airy transit circle using 19th century methods (which of course is not available since the transit circle is no longer in operation). Jc3s5h (talk) 14:11, 20 April 2015 (UTC)

There is a saying Lex non curat de minimis. That maxim has been applied here. The official conclusion from these debates is that there is nothing to worry about - time signals do indeed indicate UTC but as the time you receive depends on how far you are from your radio set and the time you adjust your watch to depends on how quickly you move nothing needs to be done. I endeavoured to explain that the legislators are aware that GMT matches UTC to within 0.9 seconds which makes it UT1. I think it is time you dropped the stick and moved on. 156.61.250.250 (talk) 14:30, 20 April 2015 (UTC)

I'm within a few feet of my radio set when I check the time, and my eyes can move very quickly to my watch. I don't use digital radio (in fact there is no digital signal where I live). I've always believed that the GMT (or BST) time signal was accurate UTC to within a few milliseconds. What's all this about 0.9 seconds? I know of no time signal that uses UT1. I would claim that "mean solar time on the Prime Meridian at Greenwich" (UT1) is NOT Greenwich Mean Time, though they might once have shared a definition. Dbfirs 18:52, 21 April 2015 (UTC)

I agree with the first part of 156.61.250.250's comment at 14:30, 20 April 2015 (UTC): "The official conclusion from these debates is that there is nothing to worry about - time signals do indeed indicate UTC but as the time you receive depends on how far you are from your radio set and the time you adjust your watch to depends on how quickly you move nothing needs to be done." The Parliament as a whole did not feel a compelling need to do anything about a difference on the order of a second. I don't agree that the Lords in the debate thought GMT was UT1. There grasp of the matter was rather shaky, so when Lord Tanlaw said in June 1997 "The question will no doubt arise as to why there is any necessity to change the law from GMT to UTC when in practice the maximum difference between the two time-scales (UTC and UT) can never exceed 0.9 of a second" we really can't take that as a statement by Tanlaw that GMT is UT1, because Tanlaw also said "The reason why this cannot be done is that UT--that is old GMT since 1975--is a general designation of all time-scales based on the Earth's rotation and therefore becomes ambiguous in that the term represents more than one time-scale." Also in December 1996 he said "Does he further agree that GMT can also be Mean Solar Time—GMST—on the Greenwich meridian? Is he aware that after 150 years Greenwich Mean Time has been removed from the list of international timescales in the current Astronomical Almanac for the reason given on sheet B5 which is, and I quote: 'Greenwich Mean Time is ambiguous'?" Clearly Tanlaw thought there were multiple possible definitions of GMT. Jc3s5h (talk) 19:57, 21 April 2015 (UTC)

The Astronomical Almanac (on page B4) says "The name Greenwich Mean Time (GMT) is not used in this Almanac since it is ambiguous and is now used, although not in astronomy, in the sense of UTC in addition to the earlier sense of UT; prior to 1925 it was reckoned, for astronomical purposes, from Greenwich mean noon (12h UT)." Who writes this rubbish? Somebody in the United States Naval Observatory, no doubt. — Preceding unsigned comment added by 156.61.250.250 (talk)

To Britons, the "Mean" in Greenwich Mean Time is the giveaway. We understand it to be the mean solar time at Greenwich, which is an observatory situated in a pleasant park south east of London. It is the mean version of the time which our sundial would show if it were transported from wherever it happens to be to the Prime Meridian. Sundials don't do leap seconds, so we are well aware that Greenwich Mean Time is not Coordinated Universal Time. — Preceding unsigned comment added by 156.61.250.250 (talk) 12:40, 22 April 2015 (UTC)

Seago, Seidelmann and Allen confirm that "UT1 ... is a precise astronomical measure of the rotation of the Earth on its axis, synonymous with mean solar time at the meridian of Greenwich, sometimes known simply as Greenwich mean time (GMT)". I note that the paper says "UTC is the legal basis for time - keeping for most countries in the world, and de - facto is the time scale used in most others". That's the difference between de jure and de facto - a live - in girlfriend may be a common law wife, but the man is free to marry someone else if he chooses.

The paper confirms "The acronym GMT survives as a common navigational synonym for UT1 despite admonitions from as far back as 1928 that astronomers are advised not to use the letters GMT in any sense for the present". And again "Such imprecise descriptions are sometimes coupled with factually incorrect statements: one example that has now multiplied into the definitions of some dictionaries and technical glossaries is the unusual claim that 'GMT was replaced by UTC in 1986' ".

In summary, "Mean solar time at Greenwich (also known as Greenwich mean time, GMT, or Universal Time, UT1) is an astronomical measure of Earth rotation referenced to the international reference meridian." 156.61.250.250 (talk) 12:57, 22 April 2015 (UTC)

I suppose this all goes to emphasise that "GMT" is not as clearly defined as I thought, but the British Government (or at least their Nautical Almanac Office) seems to think that GMT has been the same at UTC since 1972. Dbfirs 17:10, 22 April 2015 (UTC) (GMT)

Thanks for the link. I've made some changes.156.61.250.250 (talk) 11:19, 24 April 2015 (UTC)

Your claim that GMT is often considered to be identical to Coordinated Universal Time in the UK is an urban myth, like "Greenwich Meridian Time". The best way to deal with urban myths is to deny them the oxygen of publicity. I'm guessing that you live in the far north of England, but northeners are canny folk. We understand that we cannot avoid coming into contact with metres, kilogrammes, litres, degrees Centigrade and the like but we don't confuse them with yards, feet, inches, pounds, ounces, gallons and degrees Fahrenheit. We know what Greenwich Mean Time is and we know that Coordinated Universal Time is an intruder like these other units. Why, even the name is different. 156.61.250.250 (talk) 17:47, 24 April 2015 (UTC)

It's hard to determine whether most of the British population considers GMT identical to UTC, but it's obvious that they must observe UTC (in the winter) because that is the only time scales disseminated from official sources. And everyone knows they very often call the time they observe in the winter "GMT". And by "observe", I mean the time people set their timekeeping devices to, to the best of their ability. The fact that nearly all of these devices will add a few seconds per day of uncertainty, and that some timekeeping devices are hard to set any closer than a minute, does not change the time scale to which these devices are being steered by periodically resetting them. Jc3s5h (talk) 18:24, 24 April 2015 (UTC)

I'm not sure what all that was about,156.61.250.250, or why time or units should be different in London, but did you read the text of the link that you acknowledged? "The time-scale used for general purposes in the United Kingdom is Greenwich Mean Time (GMT)" and "Since 1972 January 1 the Greenwich time signals have been based on an internationally-adopted time-scale known as “Coordinated Universal Time” (UTC)". What do you conclude from these two official statements? Dbfirs 18:17, 24 April 2015 (UTC)

What everybody else (but you, apparently) concludes - Greenwich Mean Time is based on the motion of the sun and Coordinated Universal Time is based on the movement of atoms. 156.61.250.250 (talk) 18:24, 24 April 2015 (UTC)

156.61.250.250's assertion at 18:24, 24 April 2015 (UTC), "What everybody else (but you, apparently) concludes - Greenwich Mean Time is based on the motion of the sun and Coordinated Universal Time is based on the movement of atoms", just isn't so. All the astronomical authorities agree GMT is obsolete and, therfore, has no current precise definition. Coordinated Universal Time is based on both the atomic transitions and the motion of the Sun; the length of the seconds is based on atomic transitions while the insertion of leap seconds is based (through more than a century's worth of derivations) on the motion of the Sun. Jc3s5h (talk) 18:35, 24 April 2015 (UTC)

So the Astromomical Almanac, infoplease, Whitakers' Almanac and Adam Hart - Davis are all lying, are they? — Preceding unsigned comment added by 156.61.250.250 (talk) 18:42, 24 April 2015 (UTC)

In real life, it is possible to disagree on definitions without a lie being intended. Dbfirs 19:03, 24 April 2015 (UTC)

GMT is the gold standard. It's the timescale by which Coordinated Universal Time is regulated. Jc3s5h's comment that "All the astronomical authorities agree GMT is obsolete and, therfore [sic], has no current precise definition" shows how out of touch with reality he is. Dbfirs is also out of touch with reality. We are now in politically - correct 2015. The unsourced claim that Britons regard Coordinated Universal Time as identical to Greenwich Mean Time is added solely to make British people look stupid. They simply do not care about the difference between the two timescales. Dbfirs, on the other hand, will no doubt, come 1 AM on the morning of July 1, be rushing around his house moving the display on all his timepieces back by one second. This is the sort of trivia we do not insert into articles. It may well be that there are people who believe that the moon's phases are caused by the passage of the Earth's shadow across its surface. It may well be that some of those people are British. We do not record that. Still less do we write "Some British people believe that the moon's phases are caused by the passage of the Earth's shadow across its surface". 156.61.250.250 (talk) 09:26, 25 April 2015 (UTC)

Your sarcasm and ad hominem arguments are not welcome here. This page is for improvements to the article. In fact many of my clocks are corrected to "GMT" by the 60kHz transmission from Anthorn Radio Station, so I will have no need to rush around on July 1st. I have, in the past, observed the extra second being inserted to bring my clocks in line with UTC which the official link that I provided seems to regard as the same as GMT. I do agree with you that the odd second difference is not really significant for most purposes. Dbfirs 09:51, 25 April 2015 (UTC)

I have restored the version of the article that recognizes the ambiguity in GMT and have replaced the 39 year old citation to a general-interest source with a citation to the Explanatory Supplement to the Astronomical Almanac 3rd ed. (2013), a very widely cited authoritative astronomy text prepared by authors from the US Geological Survey, US Naval Observatory, Her Magesty's Nautical Almanac Office, Observatorie De Paris, National Geodeetic Survey, Jet Propulsion Laboritory, and several observatories and universities. I included a quote in the citation which specifically states that GMT can be regarded as either Universal Time (no variety specified). Jc3s5h (talk) 11:09, 25 April 2015 (UTC)

I wonder if we could compromise on saying that GMT is Universal Time without being dogmatic about which variety? Dbfirs 12:53, 25 April 2015 (UTC)

But it's not. The astronomers say it's UT1. When someone can make 40 million pounds trading in seconds on the stock exchange that's important. 156.61.250.250 (talk) 12:59, 25 April 2015 (UTC)

I agree that, historically, it was the precursor of UT1, and that astronomers continue this definition, even though they average the UT1 observations over observatories round the world, not just at Greenwich. The problem is that traders use UTC and refer to it as GMT (wrongly in your opinion). The fraction of a second difference is indeed important in automated trading. Dbfirs 13:25, 25 April 2015 (UTC)

In every system I've come across the timestamps are UTC. If you've seen different please tell me which systems were involved. 156.61.250.250 (talk) 13:38, 25 April 2015 (UTC)

I think you are correct that the normal international timestamp is "UTC" where exact timing is critical. Nevertheless it is common to associate this with GMT. (For example: [9], [10], [11], [12]). Dbfirs 14:50, 25 April 2015 (UTC)

Everybody's human. Everybody makes mistakes. I don't think it's Wikipedia's job to rub it in. It's common for people to say that we use the "Georgian calendar", but that's not so notable that we mention it in the article. Many people think we use a calender - so much so that Joe Kress made a redirect Gregorian calender>Gregorian calendar. Again, when people come to Wikipedia they want scientific information, not a discourse on common spelling mistakes. 156.61.250.250 (talk) 15:06, 25 April 2015 (UTC)

I appreciate that you think these are all mistakes, but the "error" is so common, even in official government documents (see HM Nautical Almanac Office above), that we have an obligation to report how the abbreviation "GMT" is actually used. Dbfirs 16:46, 25 April 2015 (UTC)

In a recent paper by Rots et al. (2015) it is clearly stated (several times) that GMT is continuous with UTC.

The appendix to this paper further specifies "A.8. Greenwich Mean Time (GMT) is an ill-defined timescale that nevertheless continues to persist in popular parlance as well as scientific papers. Its use is to be discouraged, but if encountered it should be interpreted as UTC, with the caveat that it is rather loosely defined as such and any assertions as to the precision of the time stamps should be regarded with caution."

The question whether GMT in UK civil timekeeping is interpreted as UTC or UT1 can easily be settled by looking how leap seconds are dealt with. If leap seconds are inserted GMT = UTC, if not, GMT = UT1. AstroLynx (talk) 14:29, 27 April 2015 (UTC)

I'm happy to confirm that GMT does not have leap seconds. The authors of the paper you cite say that GMT should be interpreted as UTC, implying leap seconds but they adduce no evidence showing that anybody ever has added leap seconds to it. They appear to know very little about their subject. They say "most terrestrial time stamps prior to 1972 should be expressed as UT (see Sect. A.9) and we recommend specifically that GMT be interpreted as UT for such dates." UTC came in long before 1972, so being logical they should recommend that GMT be treated as UT as much after 1972 as before.

In their definitions section they say as much:

"In time scale UTC the integer part of the seconds field runs from 00 to 60 (in order to accommodate leap seconds); in all other time scales the range is 00 to 59." 156.61.250.250 (talk) 16:22, 27 April 2015 (UTC)

156.61.250.250 (talk) is one of several London area IP sockpuppets of banned User:Vote (X) for Change

One of the authors in the cited paper is a UK astronomer - surely he should know something about his trade. You should also look into the paper by Hohenkerk & Hilton (2011), online here, from which I cite (p. S196)

"In many civil applications UTC is often called Greenwich Mean Time (GMT). There is no known legal connection, at least in either the UK or the US, between GMT and UTC. In the UK the name GMT does mean UTC. However, for navigation GMT has meant UT1. Thus, GMT has two meanings that can differ by as much as 0.9 s. For this reason GMT should not be used for precise purposes".

Let us know how your clock behaves on 1 July (1h BST). AstroLynx (talk) 16:48, 27 April 2015 (UTC)

@AstroLynx:, I don't think that's a valid way to show that GMT in UK civil timekeeping is UTC or UT1. There aren't all that many readily available time sources that are accurate to a tenth of a second or so. For the few that are, they could be compared to an accurate source of UTC during a time when DUT1 is large in magnitude (approaching a magnitude of 0.9 s). If the observed difference is close to 0, the source is keeping UTC; if the observed difference is close to DUT1, it's keeping UT1.

For less accurate systems, it would be necessary to observe the process used to reset the source when the inaccuracy becomes large enough to motivate the source owner to reset it. If the source owner resets to a source of UTC; the keeps UTC. Since the scientific community does not directly disseminate UT1, a source owner who wanted to reset the source to UT1 would have to find a time signal that provides some other time scale (UTC, GPS, GLONASS, etc.), look up the difference between the time signal and UT1, and reset the source taking into account the looked-up difference. Of course, most time sources are reset in private and the world does not know for certain how each owner does it, but I would be extremely surprised if any significant number of time source owners in the UK look up the difference between the time signal they are using and UT1.

The fact that a particular time source does not emit 23:59:60 hours 30 June 2015 does not prove it keeps UT1; it may just be keeping a loose approximation to UTC and the one second error will be corrected the next time the time source is reset. Jc3s5h (talk) 16:44, 27 April 2015 (UTC)

I don't live in the UK but one could check whether the coming leap second on 1 July is announced by UK media. Where I live leap seconds are inserted when appropriate (as everywhere else in the world - am I to believe that the UK is the only exception to this rule?). AstroLynx (talk) 16:53, 27 April 2015 (UTC)

If this leap second follows the pattern of previous ones, it will be widely announced in the press, radio and TV, and will be indicated by an extra "pip" in the Greenwich Time Signal. Dbfirs 20:01, 27 April 2015 (UTC)

References

• Guinot, Bernard (August 2011). "Solar time, legal time, time in use". Metrologica. 48 (4): S181–185. Bibcode:2011Metro..48S.181G. doi:10.1088/0026-1394/48/4/S08. {{cite journal}}: Invalid |ref=harv (help)
• McCarthy, Dennis; Seidelmann, P. Kenneth Seidemann (2009). TIME From Earth Rotation to Atomic Physics. Wiley-VCH.
• Seago, John H.; Seidelmann, P. Kenneth; Allen, Steve (2011). "Legislative Specifications for Coordinating with Universal Time". Advances in the Astronomical Sciences. 113: 29–50. Convenience copy at http://www.ucolick.org/~sla/leapsecs/seago.pdf

is 'GMT' EXACTLY the same C.U.T zero-hour?

CorvetteZ51 (talk) 11:21, 30 June 2015 (UTC)

There is no precise definition for GMT because the scientific community, which would provide an exact definition, no longer uses the term. Thus it isn't EXACTLY the same as anything. Also, the correct abbreviation for Coordinated Universal Time is "UTC" in all languages. It isn't clear what the original poster means by "zero-hour". Jc3s5h (talk) 18:21, 30 June 2015 (UTC)

Common useage is that GMT is exactly the same as UTC

First off, I agree that GMT should be obsoleted and replaced with UTC. It is slowly being replaced in common use but GMT is still widely used to mean UTC in a great many laws in the UK, European Union and other countries. However, the old name remains in common use and there are a huge number of systems which require sub millisecond accuracy yet still identify as using a time based on GMT. It doesn't seem logical that governments would legislate to use GMT if it would drift by up to 0.9 seconds when accurate time is vital to many systems. It's clear that the intention and practical use is that GMT is a synonym for UTC.

From 1972 UTC and UT1 were allowed to diverge by up to 0.9 seconds and at this time GMT had to follow either UTC or UT1. No one ever formally decided either way and it has never been formalised or tested in court so the de facto standard is that GMT = UTC. For people (including the UK National Physical Laboratory) to try and make a point by doggedly persisting to assert that GMT = UT1 is just bizzare and unhelpful. Indeed, the pre atomic era definition of GMT as the mean solar time at the Greenwhich Meridian is useless because 0 degrees longitude in the WGS84 datum has shifted about 100 meters from the Greenwhich Meridian line. If GMT was indeed the time at the Greenwich Meridian, then it would now either need to drift from UT1 or the old Greenwich meridian line would have to be moved. Neither of these things happened becuase people who need it this time just use UT1.

Let's just live and let GMT live on as an archaic but common name for the winter timezone in the UK and a synonym for UTC.101.98.248.252 (talk) 03:17, 3 April 2016 (UTC)

I fully agree, but how do we convince our anonymous IP editor from London and Newcastle who has some legalistic argument? Dbfirs 07:27, 3 April 2016 (UTC)

In the phrase "Let [us] just live and let", who is "us"? If you mean Wikipedia, Wikipedia should document the ambiguity. If you mean the world outside Wikipedia, consider this case: some system records time at the sub-second level, does not force users to acknowledge that the timescale kept by the system is the one and only timescale for all purposes related to the system, calls the times it records "GMT", and has important legal consequences. Perhaps the system is used to accept the filing of tax returns, or for bid submission, which have deadlines. Inevitably someone is going to miss a deadline by a fraction of a second, and sue to force the system operators to recognize the plaintiff's transaction as having met the deadline, because the system was actually keeping UTC but GMT is really UT1 and the transaction was on time according to UT1. (I don't know how such a lawsuit would turn out, but I feel confident a suit of this kind will eventually be filed.) Jc3s5h (talk) 10:15, 3 April 2016 (UTC)

The point about "Greenwich Mean Time" is that it identifies the longitude on which it is based. The term "UT1" does not. The claim that GMT is, or is believed to be, the same as UTC is absurd. Nobody who has been through secondary school could believe that a "mean" timescale could include arbitrarily inserted leap seconds. Once upon a time GMT was the mean solar time at a specific place in the Royal Park at Greenwich. Nowadays it's calculated from observation of objects in deep space. It still relates to the mean solar time at Greenwich. Greenwich is a big place, and tying it down to a specific place in the Royal Borough is unnecessary. A degree of longitude represents four minutes in time, and at the latitude of Greenwich one mile equals five seconds. So a yard is about 0.003 seconds.

Nobody can adjust a timepiece to such a degree of accuracy, although Dbfirs claims to be able to differentiate between GMT and UTC. There will never be a case on the lines described by Jc3s5h because computer timings are not that accurate - it is well known that computers cannot accommodate leap seconds. If the time is corrected by a "leap smear" the difference between GMT and UTC is never going to become an issue. The British people have decided that they are not going to let their timescale be hijacked by faceless bureaucrats in Paris. In a few weeks they may decide to turn their back on Europe altogether.

Anyone who wants to make GMT equal to UTC has an uphill task. They would have to convince the British people, their elected representatives, the House of Lords and the NPL. Being legalistic is the only way to handle a dispute. From a legal point of view, GMT was set equal to UT1 in the middle of the last century and there is no desire on the part of either astronomers or the government to change that. Supporters of change did get the idea discussed in parliament but the decision was to retain the status quo.

UTC is disseminated by radio signals but it's not a standard for anything. If you look at the digital clocks on any railway station you will see that the seconds are not aligned. Furthermore, they're designed to run to GMT rather than UTC. That's why when a leap second is broadcast the seconds change from 23:59:59 to 00:00:00 as normal. 77.98.244.158 (talk) 14:35, 3 April 2016 (UTC)

A few facts to correct here.

1. UTC is indeed the standard time used throughout the world and this is the time which is diseminated from all the accurate time sources (including the NPL in the UK) it's wrong to say "it's not a standard for anything".

2. GMT was originally the mean solar time measured at the Greenwhich meridian which was 0 degrees latitude. The Greenwich meridian is now about 100 meters away from 0 degrees latitude (in the WG84 datum which is what UT1 is based on) and GMT has not been redefined so there is no way GMT can be reckoned to be equal to UT1 anymore. GMT is now some offset from both UT1 and UTC which isn't clearly defined and no one would probably be bothered calculating because it's of no practical use.

3. Your contention that clocks in the UK run GMT is an interesting one and I would be interested to know where they get that time from because as far as I know it would be technically difficult to calculate what the actual value of the solar time at the Greenwich meridian is in order to determine GMT. My contention is that the clocks you saw at the railway station were just poorly synchronised rather than being deliberately set to GMT. I have never seen a time source anywhere that purported to diseminate GMT but if you can give an example then I'd be interested to consider it.

All that said, the name "Greenwich Mean Time" that we all know and love is widely used throughout the world and that isn't going to change any time soon. The point I am making is that for all practical purposes GMT is actually the same as UTC. I have worked with a large number of systems which require highly accurate timings and are synchronised to UTC time sources with sub millisecond accuracy yet they present their time as GMT. Quite simply there is no actual source for GMT available and it is very unlikely that anyone would bother to go to the trouble of adding the DUT1 offset to UTC to get UT1 so they could display this as GMT. The actual practice is that clocks are just synchronised to UTC and systems present it as GMT.

It may sound pedantic to be arguing over a difference of a few hundred milliseconds but I can assure you that there are numerous systems in the world to which sub second accuracy is critical to their proper operation. 101.98.248.252 (talk) 10:58, 4 April 2016 (UTC)

Thank you for your corrections here and elsewhere. Dbfirs 11:13, 4 April 2016 (UTC)

• Point 1 - When I said "UTC is not a standard for anything", fairly obviously I was referring to Britain. When the government eventually switches off the analogue radio signal the average Briton will have no access to UTC. It doesn't matter what is disseminated from Anthorn if nobody is listening, and even if they are, since timepieces are calibrated to run to mean solar time and it's impossible to split GMT and UTC when setting them they won't show UTC anyway. Dbfirs claims he is so dextrous that he can do this, but he's the only one in recorded history with that power - everyone else has to make do with stopwatches. Alternatively, he could invest in a chronometer, and then he's guaranteed accurate time (GMT of course).

• Point 2 - I think you will find that the fact that there is a calibrated brass strip in a park in Greenwich in no way inhibits the international astronomical community from decreeing that Greenwich Mean Time shall be based on another location in that pleasant suburb - it's hardly an offence under the Trades Descriptions Act now, is it? GMT doesn't need to be redefined because when UT1 came in it was made equal to GMT. If you don't believe me check with the National Physical Laboratory.

• Point 3 - If it's "technically difficult to calculate what the actual value of the solar time at the Greenwich meridian is" how did those mariners with their chronometers on the other side of the world manage it? I agree wholeheartedly that for all practical purposes GMT is actually the same as UTC. By the same token, GMT is actually the same as UT1 and UT1 is actually the same as UTC. Perm any two from three. Can you give me a screenshot of a system that uses UTC and calls it GMT? Now that would be a contravention of the Trades Descriptions Act. I advise you to report the matter to your local Trading Standards and ask them to investigate. The first thing they will ask you is "How does this system handle leap seconds?" I'm not aware of any system that can handle leap seconds. One thing you're right about - nobody would set their timepiece by adding an offset to UTC to get GMT. Since it runs on mean solar time they don't need to.

I note that all this argument avoids confronting the elephant in the room - numerous official statements by the British government that the civil time is and will continue to be Greenwich Mean Time in the winter and British Summer Time in the summer. So which is the reliable source - government statements verified by Hansard or unsupported opinion of anonymous Wikipedia editors in America, Holland and New Zealand? 77.98.244.158 (talk) 20:10, 4 April 2016 (UTC)

You've said all this before as a sockpuppet of a banned user. We don't wish to go over it all again. Dbfirs 20:17, 4 April 2016 (UTC)

When you've lost the argument pull out the WP:NPA, eh? 77.98.244.158 (talk) 20:21, 4 April 2016 (UTC)

Do you deny being that editor? ( ... and you started the personal attack ) Dbfirs 20:36, 4 April 2016 (UTC)

• Point 1 - If you want to set your time to GMT (as opposed to UTC) what do you use as a time source? I don't think there are any accurate time sources which actually diseminate GMT but I'm happy to be corrected on this. The time sources I used for precision timing (eg sub millisecond) for broadcasting, stock trading, telecommunications and other precision measurement systems in the UK were from Rugby (now Anthorn), GPS or NTP. All of these report UTC, although people routinely refer to the time setting as GMT. My contention is that every clock which is accurately synchronised to a time source is displaying a time based on UTC, even if people colloquially call it GMT.

• Point 2 - I've never disputed that the NPL say GMT = UT1 nor do I dispute that GMT is the legal time in the UK. The point I am trying to make is that the modern practical usage of the term GMT, is in fact UTC because there is no accurate source for GMT time any more. It is possible to obtain the current offset between UTC and mean solar time and set your clock using that but the vast majority of people have no inclination to do so.

• Point 3 - Allow me to offer some practical examples of people using the term GMT when they are actually using UTC. Consider the BBC, they refer to GMT editorially yet the time they use internally and diseminate on analog radio is UTC. Microsoft Windows XP also uses the term GMT even though the internal clock synchronises to within a millisecond of UTC using the Network Time Protocol (NTP). There are numerous other examples, Hotmail, Google, Microsoft, Linux have all used the name GMT interchangably with UTC. If you have a Hotmail account you can observe this by creating a calendar entry and have a look at the timezones offered, one of them is "(UTC) Greenwich Mean Time". Similar thing happens in Google calendar which offers a time zone as "GMT+00:00 London" (in winter) or any of a number of other offsets for other time zones based on GMT.

Feel free to counter with a practical example of a system that presents UT1 accurately and calls it GMT. I understand that most people don't consciously care about sub second accuracy but it is absolutely vital for commerce, telecommunications, broadcasting, banking and navigation so the accuracy of the time displayed is an important point.

As for leap seconds the question "How does this system handle leap seconds?" is answered in detail in Leap_second#Insertion_of_leap_seconds. 101.98.248.252 (talk) 09:51, 5 April 2016 (UTC)

Let's take this stage by stage. We now have consensus that GMT = UT1 = civil time in the United Kingdom. I'll work something up on that basis. 77.98.244.158 (talk) 12:30, 5 April 2016 (UTC)

Is that a consensus of one, or are you counting your many London IP addresses? Dbfirs 12:41, 5 April 2016 (UTC)

I'm evaluating the consensus from the current discussion.

In favour:

• 101.98.248.252
• myself

Against:

• none.

Unsigned comment added at 14:02, April 5, 2016‎ by 77.98.244.158

Well I am against, because GMT is no longer precisely defined and is used in different ways in different contexts. I'll allow other editors to add their votes if they wish, but many have expressed their views in this and previous sections. Dbfirs 13:18, 5 April 2016 (UTC)

Yes, I agree 100% that GMT is not precisely defined. The NPL calls it UT1, the The point I'm making is that in every day usage the de facto meaning of GMT is UTC. So far no one has come up with an example of anyone using GMT for day to day business and actually having their time accurately synchronised to a timesource other than UTC. 101.98.248.252 (talk) 11:53, 7 April 2016 (UTC)

That's what I was trying to convince our London, Newcastle and Nottingham IP editor who is now blocked. He wouldn't believe that I used to own a watch accurate enough to set to UTC rather than UT1. (That was 40 years ago when there was a more regular difference. I still own the watch, but it now shows the correct time only once a day, and it ceased to be sufficiently accurate after I dropped it.) Do you think the current version of the article accurately reflects modern usage of "GMT"? Dbfirs 13:44, 7 April 2016 (UTC)

I agree that the overwhelming majority of modern utterances and text strings that mention GMT are referring to a time scale that is, or is periodically corrected to, UTC. But this article is not just about setting modern timekeeping devices. This article could potentially be read by people who are trying to interpret old documents, or who are performing celestial navigation and are reading instructions that refer to GMT as an intermediate result. So this article can't treat GMT as a synonym for UTC. Jc3s5h (talk) 14:05, 7 April 2016 (UTC)

The first paragraph makes this clear, doesn't it? Our moving IP friend was trying to restrict GMT to UT1 only. (What a lot of time we've wasted over a difference of less than a second!) Dbfirs 14:26, 7 April 2016 (UTC)

It's claimed that some electronic devices get atomic time signals but none of them can handle leap seconds. On specific points, if you want an accurate GMT timing listen to Big Ben or get the time from clocks in public places. When people refer to Greenwich Mean Time they're talking about what the time actually is, not what some vibrating atom thinks it is. The NPL say GMT = UT1 and they're the most reliable source you could possibly have. See my observation on this at Talk:Time from NPL and the explanation at Big Ben of how the standard is set.

109.98 makes some claims about how time is measured within the BBC. How authoritative is his/her statement? Does (sh)e work for them? If (s)he's right then their clocks are different from the ones used by every other enterprise in the country. Since it is impossible for any piece of computer software to accurately present UTC (for the simple reason that no software yet devised can handle leap seconds) it's more likely than not that those companies who say their systems present GMT are actually telling the truth.

Every system that presents UT1 accurately calls it GMT because that's what it is. Nobody presents UT1 (i.e. GMT) and calls it UTC because it's not. There is one source of UTC - the NPL - and they go to great lengths to explain what the difference between the timescales is. At the end of the day we're just anonymous Wikipedia editors with no particular expertise, so why don't we just follow the reliable sources and stop the speculation? 217.44.223.15 (talk) 19:18, 28 April 2016 (UTC)

The BBC is fully aware that UTC is not GMT. They, like every member of the British public, know that GMT is based on the rotation of the Earth, and UTC is based on the vibration of atoms, which necessitates an occasional leap second when the atoms get out of sync with the sun. So please, all you Americans, Dutch and New Zealanders leave the editing of this article to the British, who know what they are talking about. As an accurate encyclopaedia, we should not even mention that some people (wrongly) think that GMT = UTC, as it only serves to confuse our readers.

217.44.223.15 (talk) 17:56, 29 April 2016 (UTC)

References

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Discrepancies between legal GMT and geographical GMT

Why is Spain legal changes so thoroughly described? Any country keeps a time zone as a result of a political decission. I wonder why there is no description on how France got to GMT+1, which is quite interesting.

The so called Presidental Order is thoroughly and poorly described. By March 1940 the decree just enforced a daylight summer time, a commonplace during economic shortages that was also observed during the Spanish Civil War (1936-39); hence the provision for its future phase out (in autumn). Even more, that decree followed similar decisions enforced in United Kingdom and France in Feb 1940. That was a bit earlier from the common practice. Governments were in a hurry as a result of the ongoing world war. All of these decisions came just before the German invasion of France.

The time zone shift silently came when clocks remained in GMT+1 during winter, again as a result of the economic shortages. Noticed, that clocks in United Kingdom also remained in GMT+1 during the winters of WWII. Also clocks in Spain had earlier experienced winter time in GMT+1. It was during the Spanish Civil War (1936-39) in the Republican zone, in the very last winter of the war: 1938-39.

This bracket "(most likely enacted to be in synchrony with Germany and Italy, with which the Franco regime was unofficially allied)" is an hoax and should be dropped out. Clocks were advanced as a result of economic shortages. Advancing clocks in United Kingdom, France and Spain results in German and Italian Mean Time. It is not any kind of synchrony. It is just geography.

Also, for this purpose it should be mentioned that Germany also moved clocks forward by that time. So there was no such synchrony.

Corsica should also be listed in the regions of France which are in physical GMT+1 with small parts of Provence, Alsace and Lorraine.

Etaoin Shdrlu (talk) 06:06, 13 November 2016 (UTC)

Semi-protected edit request on 20 December 2016

The african map is incorrect. Benin uses West Africa Time / Central European Time 195.6.224.137 (talk) 09:54, 20 December 2016 (UTC)

Not done: please provide reliable sources that support the change you want to be made. JTP ^{(talk • contribs)} 14:43, 20 December 2016 (UTC)

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Relativity

The theory of relativity seems to have been over-looked here. — Preceding unsigned comment added by 31.53.52.183 (talk) 15:32, 10 March 2018 (UTC)

You might be interested in our article on Barycentric Dynamical Time. Dbfirs 17:08, 10 March 2018 (UTC)

By the time (no pun intended) it became possible to measure time well enough to detect relativistic effects, the scientific community had abandoned the term "Greenwich Mean Time" and moved on to other the other terms mentioned in the article. Jc3s5h (talk) 03:01, 11 March 2018 (UTC)

The Article's Introduction -- possible rewrite needed

The article about GMT seems to start, rather oddly given the title thereof, with an almost anti-GMT diatribe, coming out in favor of UTC as an alternative.

Perhaps a better introduction could be written by someone who isn't "anti-GMT" or "pro-UTC". In fact, GMT is an official timezone, while UTC would be a "time standard", and a rather recent invention. While many countries use GMT as their timezone, no country uses UTC as their time zone. I think the tone of the introduction needs to be less focussed on "English-speaking" nations vs "whoever" or "us vs them" which surely doesn't belong here in Wikipedia. Therefore, both Coordinated Universal Time (UTC) and GMT have their place.

It seems that whoever wrote the introduction is more "obsessed" with "rubbishing" the importance of GMT (and comparing it to UTC), than actually explaining what it is. Don't forget this is supposed to be an article about GMT (Greenwich Mean Time) which is the official timezone used by several countries, in Europe and Africa.

Hopefully there's a well written article explaining Coordinated Universal Time (UTC), which is itself derived from International Atomic Time (TAI), somewhere else, so this article can focus upon Greenwich Mean Time (UTC).

Klf uk (talk) 21:00, 11 September 2018 (UTC)

Misuse of edit summaries

@User:2A00:23C0:7F00:C401:C176:A6FF:FD9F:5E39 I am rolling back your most recent change to Greenwich Mean Time. I have no beef with the content of your edit, but I strongly object to your misuse of edit summaries. The summary is supposed to describe the edit. This (in theory) makes it possible for an editor to review their watchlist without having to inspect a diff of each change.

You have made a total of three edits to Wikipedia mainspace, two of which were to this article, and both of those edits had summaries that didn't even try to describe the edit. The first change you made to this article was vandalism - just a bunch of words with no relevance to the article at all; the second contained a personal attack on me (and quite honestly I don't know what you are referring to with your remark about Palestinians and British, since you mentioned no particular article or diff).

To be clear: if your edit is uncontroversial or minor, it doesn't need a summary (my personal preference is that all mainspace edits should have at least a rudimentary summary). Otherwise please describe your edit in terms that enable other editors to gain a sense of the nature of your change, without comparing versions. MrDemeanour (talk) 09:43, 21 September 2018 (UTC) |}

WP:DENY
The following discussion has been closed. Please do not modify it.
Semi-protected edit request on 22 September 2018 Please remove the section "GMT in legislation" and replace with the following: GMT in legislation United Kingdom Legally, the civil time used in the UK is called still "Greenwich mean time" (without capitalisation), according to the Interpretation Act 1978, with an exception made for those periods when the Summer Time Act 1972 orders an hour's shift for daylight saving. The Interpretation Act 1978, section 9, provides that whenever an expression of time occurs in an Act, the time referred to shall (unless otherwise specifically stated) be held to be Greenwich mean time. Under subsection 23(3), the same rule applies to deeds and other instruments. "Greenwich mean time" is not defined in the Act - as so often in legislation its realisation is left to the national standards authority. In Britain this is the National Physical Laboratory, which has given it the official name of UT1. During the experiment of 1968-1971, when the British Isles did not revert to Greenwich Mean Time during the winter, the all-year British Summer Time was called British Standard Time (BST). Other countries Several countries define their local time by reference to Greenwich Mean Time. Some examples are: Belgium: Decrees of 1946 and 1947 set legal time as one hour ahead of GMT. Ireland: Standard Time (Amendment) Act, 1971, section 1, and Interpretation Act 2005, part iv, section 18(i). Canada: Interpretation Act, R.S.C. 1985, c. I-21, section 35(1). This refers to 'standard time' for the several provinces, defining each in relation to 'Greenwich time', but does not use the expression 'Greenwich mean time'. Several provinces, such as Nova Scotia (Time Definition Act. R.S., c. 469, s. 1), have their own legislation which specifically mentions either "Greenwich Mean Time" or "Greenwich mean solar time". Realisation of Greenwich Mean Time In the UK, UTC+0 is disseminated to the general public in winter and UTC+1 in summer. BBC radio stations broadcast the "six pips" of the Greenwich Time Signal. It is named from its original generation at the Royal Greenwich Observatory. It is a close approximation (within 0.9 seconds) to either Greenwich Mean Time or British Summer Time as appropriate for the time of year. Specialist services are available to those who require accurate GMT. A coded signal giving the offset between GMT and UTC is broadcast by the National Physical Laboratory in the U K. See Time from NPL (MSF)#Slow code. An audible GMT time signal is provided in the U S by the National Institute of Science and Technology (see WWV (radio station)) and in China by the Chinese Academy of Sciences (see BPM (time service)). The service is available from other providers. References To anon IP 89.243.9.168 who proposed the above: the pips of the Greenwich Time Signal are precisely UTC, as already stated in the article. Calling them a "close approximation" just introduces further confusion. Dbfirs 12:19, 22 September 2018 (UTC)

A Commons file used on this page has been nominated for deletion

The following Wikimedia Commons file used on this page has been nominated for deletion:

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That "current time" in the box on the right, is totally off its hinges.

It certainly isn't 17:58 anywhere right now (time in Denmark right now is 20:35). — Preceding unsigned comment added by 212.237.134.116 (talk) 18:35, 17 September 2019 (UTC)

You may need to click the "Refresh" link alongside the time. Rosbif73 (talk) 18:48, 17 September 2019 (UTC)

Zulu time

In this edit Mndata (talk · contribs) made the following change:

English speakers often use GMT as a synonym for Coordinated Universal Time (UTC), also known as Zulu time. [Addition underlined]

The chief problem is that a significantly different issue is stuffed into a sentence about something else.

The second problem is the cited source does not support the claim. There has been a great deal of contention in this article, and editors not familiar with the topic have often found themselves having to repair vandalism. All edits should be supported by reliable sources.

The third problem is that there are several other names for the same time zone, such as Western European Time. If we tried to stuff all of them into the first paragraph the paragraph becomes unwieldy.

The fourth problem is that the sentence is unclear; is Zulu a synonym for UTC, GMT or both?

I think we should discuss where in the article we should mention Zulu and what we should say about it. Jc3s5h (talk) 14:16, 15 April 2019 (UTC)

... We could cite J. M. Pasachoff's & M L Kutner's University Astronomy: "Astronomers often keep track of events according to the standard solar time that corresponds to the Greenwich time zone. This is called G.M.T. (Greenwich Mean Time), U.T. (Universal Time), or Z (which is colloquially called Zulu Time)."

... or Webster's New World Telecom Dictionary says, for GMT: "Civilian terminology for Universal Coordinated Time (UTC), which is known as Zulu Time (Z) for military and aviation purposes." and for Zulu time: "Also known as Universal Coordinated Time (UTC) and Greenwich Mean Time (GMT), the latter of which is used for civilian purposes. The world is divided into 25 integer time zones, each of which is identified by a letter in the English alphabet. (Note: There is no time zone J.) The clock at Greenwich, England, is the standard clock for international time reference, and is referred to in military and aviation parlance as the Zulu (Z) Time, as it is in the time zone designated by the letter Z." Dbfirs 11:42, 18 July 2019 (UTC)

The problem I see with Webster's New World Telecom Dictionary is it implies that Z or zulu is the term exclusively used by the military and aviation communities, and civilians don't use it. But everybody uses "GMT", "UTC", and "Z", it's just a matter of degree. Jc3s5h (talk) 13:19, 18 July 2019 (UTC)

Nobody really uses Z in that sense (with other letters), only this sense is used 2019-11-01T19:32:40Z (which means it MUST be in UTC+-00:00). Used in logs, etc. Only Z is used, nothing else. Again UTC IS not time Zone, while Zulu IS. 94.29.6.63 (talk) 17:48, 17 June 2020 (UTC)

References

RFC: Explain obsolete nature of GMT in the lead?

There is no consensus for these edits to the article.

Cunard (talk) 00:51, 21 October 2018 (UTC)

The following discussion is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

---

Shall the lead of the article Greenwich Mean Time explain the obsolete nature of the terms "Greenwich Mean Time" and "GMT", given that they are no longer precisely defined by the scientific community? Jc3s5h (talk) 22:01, 11 September 2018 (UTC)

In fact, GMT is not obsolete. It is used to define time zones, while, Coordinated Universal Time (UTC) is a scientific time standard based upon International Atomic Time (TAI). That has its own article, and I do not state that they are equivalent, in fact, GMT's role is primarily that of defining a time zone, and its name is helpful because it inherently explains why the location was chosen. Klf uk (talk) 22:32, 11 September 2018 (UTC)

Really? Most time zones now appear to be based on UTC, hence why most time zones are defined by their UTC offset, not GMT. Davidbuddy9💬 12:36, 12 September 2018 (UTC)

Discussion of lead of "Greenwich Mean Time"

I believe the series of edits by Klf uk (talk · contribs) attempts to put GMT on a par with UTC, but this is misleading because UTC is precisely defined by the scientific community and precisely disseminated by many precise facilities, such as NTP servers operated by national time laboratories, GPS satellites, and standard frequency and time signal services. GMT is not precisely disseminated and is inherently ambiguous. Jc3s5h (talk) 22:01, 11 September 2018 (UTC)

Coordinated Universal Time (UTC) is scientifically defined, however that doesn't make GMT obsolete or misleading. In fact, you can trace back UTC to GMT (currently, via TAI). In fact, if you consider it more carefully, the meridian for UTC is not very far off that of GMT, and that is for good reason. GMT is currently used, please do not disparage it. It has current as well as historical significance, although it perhaps has not been scientifically defined in the same way that TAI has been since its inception. Klf uk (talk) 22:32, 11 September 2018 (UTC)

In 2007, the United States changed its time zones' reference from "the mean solar time of ... Greenwich" to "Coordinated Universal Time". For the applicable old and new paragraphs in 15 USC 261(a) and the legislative record see 15 USC 260–267. — Joe Kress (talk) 01:28, 12 September 2018 (UTC)

For a good discussion of GMT vis-a-vis UTC see Legislative specifications for coordinating with Universal time. — Joe Kress (talk) 03:05, 12 September 2018 (UTC)

As that reference makes clear, both UTC and UT (GMT) are well defined and neither is "better" than the other. They simply have different aims: UTC uses the same invariant (atomic) definition of the second as TAI, whereas UT is aligned with observed solar time. The difference is ironed out using leap seconds. In the vast majority of cases, the de jure definition of time zones is relative to UTC, i.e. the use of leap seconds is implied by law. Some countries do still legally define their time zone relative to GMT, though in most cases de facto civil timekeeping is based on UTC with its leap seconds. Rosbif73 (talk) 13:08, 12 September 2018 (UTC)

I'm sorry, but GMT is not well-defined. There have been numerous definitions of GMT over the years, all different. At times there have been more than one definition in use at the same time, for different purposes. Please read this: [13]. UT is currently well-defined, but GMT is not UT.

"Some agencies currently assert that GMT is currently used as a synonym for UTC. Below is a history of meanings which have been applied to GMT. There is no current definition of GMT which is authoritative in all contexts."

The point is that when using GMT to give the time of an event, you have to state what kind of GMT you are using. If you don't do that, then you have failed to give the time of the event. You can't just assume the current 'meaning' of GMT, because there isn't one. And assuming simply UT doesn't help, because UT has also had a history of definitional changes without corresponding name-changes.

Imagine you are writing a history of (say) the Battle of Jutland. You give the time at which Galatea opened fire as 14:28, and the time of the first hit on Galatea by the Elbing at 14:36. But are these times using GMT as it was at the time of writing, or at the time of the events themselves? Or should the reader assume the version of GMT to which their own watch is synchronized? This might matter if the reader were comparing two accounts of the battle by different authors.

The point is, people must stop changing the definitions of timescales without also changing their names. The politicians and diplomats who define timescales still haven't understood this; removing leap-seconds from UTC would be exactly such a change. MrDemeanour (talk) 15:56, 12 September 2018 (UTC)

It's understood that UT generally corresponds to the mean solar time near the Greenwich meridian, with the Sun being close to the meridian at 12:00 UT. The best refinement of this understanding I've found is in the proceedings of the XVIth General Assembly of the International Astronomical Union, Grenoble, France.[14] On page 27 of the PDF it states "Recognize that UT may be used ln the place of UTO, UTI, UT2 and UTC ln cases where the distinction between them is not needed...." So we might say that UT is well-defined as the expression to use when the distinctions among UT0, UT1, UT2, and UTC are not important for the purpose at hand. 16:23, 12 September 2018 (UTC)

Thanks for all the great comments, I can see there's much more to this than I had realised. I think that there has been several occurrences of changing the definition of GMT, without enough care, scientifically, going into explaining the change or giving the scientific community a way to name each "version". This has caused much anguish and inconvenience, this is reflected in people's reaction which is to move away from it (GMT) in many parts of the world, thus the creation of Coordinated Universal Time. I do see the point(s) being made repeatedly that this is not only infuriating for people who wish to accurately record events, but it is also very unhelpful, and the lack of clarity has tended to push people towards more "well defined" and understood measures such as UTC, however that also isn't perfect, seemingly.

I appreciate that at the current time (pardon the pun) it isn't useful to have a debate about whether GMT is "current" on the main GMT wikipedia page (or useful given the international preference for UTC) however I would love it if someone could have a go at summarising the issues described in this Talk and place it on the GMT page, as otherwise it simply isn't clear for a lay-person (me in this case) why the article comes across as "anti-GMT". In fact, it's very useful to see that what the community wants and needs are clear standards, and for politicians to back off from messing around with standards without realising the consequences of doing so. Almost, I would imagine a new section called "lessons learned" or something (assuming that there's a consensus that lessons have been learned, that is??)

The current definition for better or worse of GMT does indeed, though, seem to be Universal Time. Is there a scientific convention (yet?) for updating any kind of internationally defined unit, or does it vary greatly by unit? I'm thinking of pounds / lbs (avoirdupois vs troy) -- a simple label helps here, whereas for long / lat we use (WGS84 vs ETRS89). Perhaps the current system, i.e. where GMT is equivalent to UT, should be called GMT2018 or whenever that was officially decided (if that can be deduced from any official internationally recognised documents?) Just a thought, as people would and do come to this page for guidance, and if there's already a precedent for correctly documenting and recording "versions" of units or measures, should / could the current best practice not also be applied to GMT?

Klf uk (talk) 20:32, 17 September 2018 (UTC)

I would say the consensus in the scientific community, and which is also compatible with those governments that have defined their civil time as an offset from UTC, is to use UT if the time doesn't have to be more precise than a few seconds. If it does have to be precise to the second or better, use UT0, UT1, UT2, or UTC as appropriate. If quoting an old source that uses GMT, in square brackets, put UT if the sun is near the zenith at Greenwich about 12:00. If the source is so old that the sun is near the zenith at Greenwich near 00:00, put Greenwich Mean Astronomical Time in square brackets.

As for the UK which clings to "Greenwich Mean Time" in their official definition of civil time, I'm a volunteer, and I don't volunteer to mess with that. Jc3s5h (talk) 21:17, 17 September 2018 (UTC)

I have reviewed the series of edits. This appears to me to be WP:POVPUSH. I prefer the reverted version of the article as it now stands. ~Kvng (talk) 15:53, 9 October 2018 (UTC)

WP:DENY
The following discussion has been closed. Please do not modify it.

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The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

To MrDemeanour: About what you wrote: "I'm sorry, but GMT is *not* well-defined." I don't think you need to be sorry. It's not your fault. Toddcs (talk) 16:35, 14 March 2021 (UTC)

Its effective successor

In this edit User:DemonLover456 made the following change:

English speakers often use GMT as a synonym for its effective successor Coordinated Universal Time (UTC). For navigation, it is considered equivalent to UT1 (the modern form of mean solar time at 0° longitude); but this meaning can differ from UTC by up to 0.9 s. The term GMT should not thus be used for certain technical purposes requiring precision. [addition underlined]

Before, the paragraph made sense, explaining that sometimes GMT means UTC, but sometimes it means UT1. The addition creates a contradiction; after the change, the paragraph says that for dates after about 1960, GMT always means UTC, but sometimes after 1960, it means UT1. Therefore I reverted the change. Jc3s5h (talk) 15:59, 30 November 2020 (UTC)

References

GMT never means UTC. There is only UTC now. UTC+00:00 is called GMT time zone, which is just one of many. Okay? GMT that used Greenwich meridian is not used anymore, as it is not 0° meridian, IERS Reference Meridian is and used for UTC. 79.104.6.76 (talk) 16:52, 28 April 2021 (UTC)

Opening sentence still causing difficulties

Maybe the problem is that we are trying to satisfy two different readerships? I removed the word "historically" to leave the current opening sentence to read

Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London

the citation supports it, and I think it has the right balance for a general readership. HMNA/USNO Astronomical Almanac dances around the subject, which I suspect is a political minefield:

Universal Time (UT):

a generic reference to one of several time scales that approximate the mean diurnal motion of the Sun; loosely, mean solar time on the Greenwich meridian (previously referred to as Greenwich Mean Time). In current usage, UT refers either to a time scale called UT1 or to Coordinated Universal Time (UTC); in this volume, UT always refers to UT1. UT1 is formally defined by a mathematical expression that relates it to sidereal time. Thus, UT1 is observationally determined by the apparent diurnal motions of celestial bodies, and is affected by irregularities in the Earth's rate of rotation. UTC is an atomic time scale but is maintained within 0s.9 of UT1 by the introduction of 1-second steps when necessary.

The Almanac does not define Greenwich Mean Time directly, there is only this loosely [sic], mean solar time on the Greenwich meridian (previously referred to as Greenwich Mean Time). So, speaking 'loosely' of course, is our lead sentence good enough or does it need to be changed again? (Bear in mind wp:LEAD: a succinct summary of the body, not a rewrite or alternative version of it). --John Maynard Friedman (talk) 15:51, 17 September 2021 (UTC)