What Is a Leap Year?

A leap year adds one extra day — February 29 — to keep the calendar year aligned with Earth's orbit around the Sun, which takes approximately 365.2422 days rather than an even 365.

The rule

• A year is a leap year if it is divisible by 4.
• Exception: a year divisible by 100 is not a leap year...
• ...unless it is also divisible by 400, in which case it is.

So 2024 and 2028 are leap years, 1900 was not, and 2000 was. Under this rule the calendar drifts from the solar year by only about one day every 3,300 years.

Why it matters for dates

Leap years affect week numbers, day-of-year counts, and the exact gap between two dates that span February. Every calculation on this site accounts for leap years automatically — see the Date Calculator and Date Difference tools.