Life & Career · 闰年（Leap Year） · Updated 2026-10-25
What Is Leap Year?
Definition
Earth's orbit takes ~365.2422 days, so a 365-day calendar drifts ~0.2422 days per year; adding a day every 4 years is the main correction. But that over-corrects by ~0.0078 days per year, hence the century rule: years divisible by 100 are not leap years unless divisible by 400. So 1900 was not a leap year, 2000 was. The three-tier rule keeps the average Gregorian year within ~26 seconds of the true year — one day of drift in ~3300 years.
Formula
Leap ⇔ (year % 4 == 0 AND year % 100 != 0) OR year % 400 == 0
Worked Example
2024 is a leap year (divisible by 4, not a century year); 2100 is not (÷100 but not ÷400); 2400 is (÷400). People born on 29 Feb legally celebrate on 28 Feb or 1 Mar in common years.
Common Misconception
The "every 4 years" rule misleads on century years: mistaking 2000 as common or 2100 as leap are the two classic errors. Age and interest calculations that ignore 29 Feb can disagree by a day in 2024.
Matching Calculators
Related Terms
Keep Exploring
Browse the full glossary, or head back to the homepage for 1,435 free online calculators.
Back to glossary