About 5 million people worldwide were born on February 29. Most years their birthday doesn't exist — and jurisdictions disagree on what to do about it. The math and the conventions, in three minutes.
The leap-year rule
A leap year has 366 days instead of 365. The solar year is ~365.2425 days, so adding a leap day roughly every 4 years keeps the calendar aligned with the seasons. The exact rule:
- Divisible by 4 → leap year.
- Except divisible by 100 → not a leap year.
- Except divisible by 400 → leap year.
So 2000 was a leap year. 1900 wasn't. 2100 won't be. 2400 will be. This Gregorian refinement, adopted in 1582, corrects the Julian calendar's drift — accurate to 1 day every ~3,000 years.
When do you "turn one"?
For someone born Feb 29, the legal birthday in non-leap years varies:
- US (most states) — March 1. You're 21 on March 1 in a non-leap year.
- UK — March 1.
- Hong Kong, New Zealand — February 28.
- Ireland — Feb 28 for legal purposes.
- Taiwan — Mar 1.
Internationally the majority convention is March 1. Our age calculator follows this rule.
The celebration question
Legal conventions aside, leaplings usually pick one of three approaches: celebrate on Feb 28, celebrate on March 1, or only celebrate in actual leap years (once every 4 years). None is "correct" — it's a personal choice. Some leaplings lean into the uniqueness and claim to be "8 years old" at 32.
Leap-year aware. Handles Feb 29 birthdays consistently using the March 1 convention.