Calendar year

A calendar year begins on the New Year's Day of the given calendar system and ends on the day before the following New Year's Day, and thus consists of a whole number of days.

To reconcile the calendar year with the astronomical cycle (which has a fractional number of days) certain years contain extra days ("leap days" or "intercalary days"). The Gregorian year, which is in use in most of the world, begins on January 1 and ends on December 31. It has a length of 365 days in an ordinary year, with 8760 hours, 525,600 minutes, or 31,536,000 seconds; but 366 days in a leap year, with 8784 hours, 527,040 minutes, or 31,622,400 seconds. With 97 leap years every 400 years, the year has an average length of 365.2425 days. Other formula-based calendars can have lengths which are further out of step with the solar cycle: for example, the Julian calendar has an average length of 365.25 days, and the Hebrew calendar has an average length of 365.2468 days. The Lunar Hijri calendar ("Islamic calendar") is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. The astronomer's mean tropical year, which is averaged over equinoxes and solstices, is currently 365.24219 days, slightly shorter than the average length of the year in most calendars.

A year can also be measured by starting on any other named day of the calendar, and ending on the day before this named day in the following year. This may be termed a "year's time", but is not a "calendar year".

Quarter year

"Fourth quarter" redirects here. For the sports term, see Comeback (sports).

The calendar year can be divided into four quarters, often abbreviated as Q1, Q2, Q3, and Q4. Since they are three months each, they are also called trimesters. In the Gregorian calendar:

• First quarter, Q1: January 1 – March 31 (90 days or 91 days in leap years)
• Second quarter, Q2: April 1 – June 30 (91 days)
• Third quarter, Q3: July 1 – September 30 (92 days)
• Fourth quarter, Q4: October 1 – December 31 (92 days)

In some domains, weeks are preferred over months for scheduling and reporting, so they use quarters of exactly 13 weeks each, often following ISO week date conventions. One in five to six years has a 53rd week which is usually appended to the last quarter. It is then 98 days instead of 91 days long, which complicates comparisons.

In the Chinese calendar, the quarters are traditionally associated with the 4 seasons of the year:

• Spring: 1st to 3rd month
• Summer: 4th to 6th month
• Autumn: 7th to 9th month
• Winter: 10th to 12th month

Quadrimester

The calendar year can also be divided into quadrimesters (from French quadrimestre), lasting for four months each. They can also be called the early, middle, or late parts of the year. In the Gregorian calendar:

• First quadrimester, early year: January 1 – April 30 (120 days or 121 days in leap years)
• Second quadrimester, mid-year: May 1 – August 31 (122 days)
• Third quadrimester, late year: September 1 – December 31 (121 days)

Semester

The calendar year can also be divided into semesters, lasting six months each and often being abbreviated as S1 and S2. In the Gregorian calendar:

• First semester, S1: January 1 – June 30 (181 days or 182 days in leap years)
• Second semester, S2: July 1 – December 31 (184 days)

See also

• Academic term – Subdivision of the academic year at educational institutions
• Calendar reform – Significant revision of a calendar system
• Common year – Calendar year with 365 days
• Fiscal year – One-year term for government and business financial reporting
• ISO 8601 – International standards for dates and times
• ISO week date – Leap week calendar system
• Leap year – Calendar year containing an additional day
• Model year – Production date of a commercial product
• Tropical year – Period of time for the ecliptic longitude of the Sun to increase 360°
• Seasonal year – Time between successive recurrences of a seasonal event