With the arrival of 2008 starts a year is a leap, and this is a period of duration of 366 days instead of 365 a year together. That extra day is often added at the end of the month shorter fechándose as February 29.

The duration of the year (the time it takes the Earth to make one complete revolution of the Sun) is not exactly 365 days, but approximately 365 days and fourth day. By made up the Gregorian calendar, the current force, it took an average duration of the solar year of 365.2425 days, known as the Gregorian year. The way to adjust the gap to 1 / 4 of day was established (as in the previous Julian calendar) as leap years multiples of four, but as the Gregorian year is a bit shorter than the real or tropical year, three are deleted julianos leap every 400 years, in years divisible by 400 non-secular, such as 1700, 1800, 1900 and 2100, while that leap if they are 1600, 2000, etc.. that are divisible by 400.

This day is added to correct the gap that exists with the actual duration of the years: 365 days and 6 hours approximately. This makes it corrected every four years (years multiples of four) that accumulate 24 hours.

The Julian calendar considered leap years divisible four. This year Julian lasts 365 days +1 / 4 = 365.25 days (rather than the tropical year which lasts 365.2422 days).

The rule for leap years according to the Gregorian calendar is:

A leap year is if it is divisible by 4, except the last of a century (those divisible by 100), that to be leap, it must be divisible by 400.

That is the years that are divisible by 4 are leap, but will not be leap if they are divisible among 100 (as the years 1700, 1800, 1900 and 2100) unless they are divisible by 400 (as the years 1600, 2000 or 2400 ). In 400 years there should be 97 leap years, thus the year of the Gregorian calendar is very similar to the solar year. Thus, the Gregorian year lasts 365 days +1 / 4 -1/100 +1 / 400 = 365.2425 days (rather than the tropical year which lasts 365.2422 days).

Since the error is 0.0003 days per year, it would seem that after three thousand years will be a day of accumulated error. But in reality we do not know exactly when the mistake to come one day. The figure of 365.2422 days a tropical year is not entirely accurate, because the duration of the tropical year, as the speed of rotation of the earth, change over the centuries, and in a manner that is not completely predictable.

There are versions indicating that that day will be deducted by the year 3600, which may not be a leap year