Time scales and civil time
Leap year
Also: schrikkeljaar
A solar year lasts about 365.2422 days. To absorb that difference the Gregorian calendar adds a leap day every four years, with one exception: century years are only a leap year if divisible by 400 (2000 was, 1900 and 2100 are not).
This keeps the calendar aligned with the seasons over the centuries.