A leap year occurs every 4 years, except for years divisible by 100 but not by 400. This extra day, February 29th, is added to align the calendar year with the astronomical year.
Why do we need leap years?
The Earth takes approximately 365.2422 days to orbit the Sun (a tropical year), but the standard calendar year has only 365 days. Without leap years, the calendar would drift by about 1 day every 4 years, and after 100 years the calendar would be off by nearly a month. Leap years compensate for this drift by adding an extra day every 4 years (on average), keeping the calendar aligned with the seasons.
The leap year rules
- A year is a leap year if it's divisible by 4.
- EXCEPT: if it's divisible by 100, it's NOT a leap year.
- EXCEPT: if it's divisible by 400, it IS a leap year.
So 2000 was a leap year (divisible by 400), but 1900 was not (divisible by 100 but not 400). 2024 was a leap year, and 2028 will be too. The year 2100 will NOT be a leap year, which will be the first time this exception applies since 1900.
The impact of leap years
Leap years affect everything from birthdays (people born on February 29 only have a "real" birthday every 4 years) to financial calculations (interest accrual over a leap year is different from a standard year) to software (date libraries must handle February 29 correctly). Our Age Calculator automatically accounts for leap years when computing your exact age.