Leap Years, Leap Centuries

Century years, such as 1900 2000, are leap years if they are evenly divisible by 400. The basic rules for calculating leap years are:

The need for leap years is due to the fact that the actual length of a year is 365.242 days, not 365, as is commonly stated. To account for this, an extra day is added as February 29th on years that are evenly divisible by 4 (eg. 1992).

Since the year is slightly less than 365.25 days long, adding an extra day every 4 years results in about 3 extra days being added over a 400-year period. For this reason, 1 out of every 4 century years also needs to be a leap year.

Using this arrangement a year has 365.2425 days on the average.