Leap year
Gregorian calendar

In the Gregorian calendar, the standard calendar in most of the world, almost every fourth year is a leap year. Each leap year, the month of February has 29 days instead of 28. Adding one extra day in the calendar every 4 years compensates for the fact that a period of 365 days is shorter than a tropical year by almost 6 hours. However, this correction is excessive and the Gregorian reform modified the Julian calendar's scheme of leap years as follows:

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the years 1600 and 2000 are.

Whereas the Julian calendar year incorrectly summarised Earth's tropical year as 365.25 days, the Gregorian calendar makes these exceptions to follow a calendar year of 365.2425 days. This more closely resembles a mean tropical year of 365.2422 days. Over a period of four centuries, the accumulated error of adding a leap day every 4 years amounts to about 3 extra days. The Gregorian calendar therefore omits 3 leap days every 400 years, which is the length of its leap cycle. This is done by omitting 29 February in the 3 century years (multiples of 100) that are not multiples of 400. The years 2000 and 2400 are leap years, but not 1700, 1800, 1900, 2100, 2200, and 2300. By this rule, an entire leap cycle is 400 years which total 146,097 days, and the average number of days per year is 365 + 1 ⁄ 4 − 1 ⁄ 100 + 1 ⁄ 400 = 365 + 97 ⁄ 400 = 365.2425. This rule could be applied to years before the Gregorian reform to create a proleptic Gregorian calendar, though the result would not match any historical records.

The Gregorian calendar was designed to keep the vernal equinox on or close to 21 March, so that the date of Easter (celebrated on the Sunday after the ecclesiastical full moon that falls on or after 21 March) remains close to the vernal equinox. The " Accuracy " section of the " Gregorian calendar " article discusses how well the Gregorian calendar achieves this design goal, and how well it approximates the tropical year.
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