Leap Years Explained

A leap year has 366 days instead of 365, with an extra day added to February (the 29th). Leap years keep the calendar aligned with the Earth’s orbit around the Sun, which takes about 365.2422 days — not a whole number.

The rule

A year is a leap year if it is divisible by 4 — except years divisible by 100, which are not leap years, unless they are also divisible by 400, which are. So 2000 was a leap year, but 1900 and 2100 are not.

• Divisible by 4 → leap year…
• …unless divisible by 100 → not a leap year…
• …unless divisible by 400 → leap year after all.

Why the extra rules

Adding one day every four years slightly overcorrects, because the orbital year is a little under 365.25 days. Skipping the leap day in most century years trims the excess, and the divisible-by-400 exception fine-tunes it further. The result keeps the calendar accurate to within one day over roughly 3,000 years.

This is the Gregorian calendar, introduced in 1582 to fix drift that had accumulated under the older Julian calendar, which used the simpler every-four-years rule.