The Gregorian calendar adds February 29 every four years, except in century years unless they divide by 400. This prevents seasons from drifting slowly over centuries.
- Years divisible by 4 are leap years.
- Centuries (like 1900, 2100) skip the leap day unless divisible by 400 (like 2000, 2400).
- Leap seconds may also appear, keeping atomic seconds aligned with Earth day lengths.