Every four years February stretches to 29 days and we call it a leap year. It feels like a quirk, but it’s a careful fix for a real problem: the Earth doesn’t orbit the Sun in a tidy whole number of days. Here’s the rule, the exception most people miss, and why it matters for counting age and dates.
Why we need leap years at all
A full orbit of the Sun — one solar year — takes about 365.2422 days, not exactly 365. That extra quarter-day a year doesn’t sound like much, but ignore it and the calendar drifts against the seasons by roughly a day every four years. Left unchecked for a few centuries, midsummer would slide into what the calendar calls winter. Adding a day every four years soaks up those accumulated quarter-days and keeps the calendar locked to the seasons.
The rule (including the part people forget)
The Gregorian calendar uses a three-part rule:
- A year divisible by 4 is a leap year — so 2024 and 2028 are.
- Except years divisible by 100, which are not leap years — so 1900 was not.
- Unless they’re also divisible by 400, which makes them leap years after all — so 2000 was.
Why 365.2422, not 365.25?
Adding one day every four years assumes the year is exactly 365.25 days. It’s slightly less, so the simple rule overcorrects by a tiny amount. The century exceptions (skipping three leap years every 400 years) trim that overcorrection down, leaving an error of less than a day across thousands of years — accurate enough that we won’t need another fix for a very long time.
What this means for age and date calculations
Leap years are exactly why you can’t turn an age into a number of days by multiplying years by 365 — you’d be short by one day for every leap year in the span. Anyone born on 29 February has a “real” birthday only once every four years, though they typically celebrate on 28 February or 1 March in common years. A reliable age calculator sidesteps all of this by counting actual calendar dates, so every leap day is automatically included.
Leap year quick facts
- The next leap years are 2028, 2032 and 2036.
- 2000 was a leap year; 1900 and 2100 are not.
- People born on 29 February are sometimes called “leaplings” or “leap-year babies.”
- There are roughly 97 leap years in every 400-year cycle.