The Complete Gregorian Leap Year Formula
A solar year—the time it takes Earth to orbit the Sun—is approximately 365.2422 days, not exactly 365.25 days.
To prevent calendar drift over centuries, Pope Gregory XIII instituted the Gregorian calendar in 1582 with three strict rules:
- Every year that is evenly divisible by 4 is a leap year...
- ...except for end-of-century years (ending in 00), which are common years...
- ...unless the century year is also evenly divisible by 400.
This is why 2000 was a leap year (divisible by 400), but 1900 was not, and 2100 will not be. For a full historical exploration, read our article Understanding Leap Years.
Want to see how leap years affect your exact birthdate and age? Use our Age Calculator.