Leap years and your real age
If you were born on 29 February, when do you legally turn another year older? And how do leap days quietly change ages calculated in days?
4 min readReviewed May 1, 2026
Quick answer
Leap years add a 29 February every four years (with a small exception every 100 and 400). They shift day-counts by one and create the famous "leapling" birthday edge case.
The leap year rule, in full
A year is a leap year if it's divisible by 4 — except century years, which must also be divisible by 400. So 2000 was a leap year, 1900 was not, 2024 was, 2100 will not be.
Most people only ever notice the every-four-years pattern. The 100/400 exception keeps the calendar aligned to the solar year over centuries.
Where leap days affect age
- Age in days: any span that crosses a 29 February gains one extra day.
- Leapling birthdays: people born on 29 February usually mark birthdays on 28 Feb or 1 Mar in non-leap years; legal age still ticks up on 1 March in most jurisdictions.
- Average year length: 365.2425 days — close to 365.25, but not quite.