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:
- Years divisible by 4 are leap years, but
- Years divisible by 100 are not leap years, but
- Years divisible by 400 are leap years.
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.