Adding an extra day to the calendar every four years compensates for the fact that a solar year is almost 6 hours longer than 365 days ... However, some exceptions to this rule are required since the duration of a solar year is slightly less than 365.25 days. Years which are evenly divisible by 100 are not leap years, unless they are also evenly divisible by 400, in which case they are leap years For example, 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not. Going forward, 2100, 2200, 2300, 2500, 2600, 2700, 2900, and 3000 will not be leap years, but 2400 and 2800 will be. By this rule, the average number of days per year will be 365 + 1/4 − 1/100 + 1/400 = 365.2425, which is 365 days, 5 hours, 49 minutes, and 12 seconds ... The marginal difference of 0.000125 days means that in around 8,000 years, the calendar will be about one day behind where it is now. But in 8,000 years, the length of the vernal equinox year will have changed by an amount which cannot be accurately predicted
info ganked from wikipedia :D
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