Calendar year

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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.

The Gregorian calendar year, which is in use as civil calendar in most of the world, begins on January 1 and ends on December 31.^[1] It has a length of 365 days in an ordinary year but, in order to reconcile the calendar year with the astronomical cycle, it has 366 days in a leap year. With 97 leap years every 400 years, the Gregorian calendar 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.^[a] 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 calendar 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.^[2] This may be termed a "year's time", but is not a "calendar year".

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