Leap Year Checker (with rule explanation)

The Gregorian leap year rule

A year is a leap year if it's divisible by 4 — except for centuries (divisible by 100), which are leap years only when also divisible by 400. So 1900 is NOT a leap year (divisible by 100, not 400), but 2000 IS (divisible by 400). 2024 is a leap year (divisible by 4); 2100 will not be.

This rule keeps the calendar in sync with Earth's actual orbit (~365.2425 days). A simple every-4-years rule would drift by about 3 days every 400 years; the century exception corrects most of that drift, leaving an error of only about 1 day in 3,300 years.

Why we need leap years at all

Earth orbits the Sun in about 365.2422 days — not exactly 365. Without periodic correction, calendar dates would slowly drift relative to the seasons. After 100 years of plain 365-day years, the calendar would be ~24 days behind the equinoxes and seasons would appear to shift across the calendar.

The Julian calendar (45 BC) added a leap year every 4 years, which was close but slightly over-corrected. Pope Gregory XIII's reform in 1582 introduced the century rule, producing the modern Gregorian calendar still in use today.

Leap year quirks

Feb 29 birthdays: people born on Feb 29 ('leaplings') celebrate every 4 years on the actual date, or on Feb 28 / Mar 1 in regular years. Legally, most jurisdictions treat them as Feb 28 birthdays for age-determined purposes.

Some software has historically treated all century years as non-leap (a bug introduced when implementing the simple every-4 rule). 2000 worked correctly because it's div by 400, but 2100 will reveal latent bugs in any system that hasn't tested ahead.

Astronomical leap year is slightly more complex: solar year is ~365.2422 days, lunar months are ~29.53 days. The Hebrew, Islamic, and Chinese calendars handle leap concepts differently, often by adding a whole leap month rather than a single day.