Leap day birthdays: how age works if you were born on 29 February

If you were born on 29 February, your birthday appears on the calendar only once every four years, but you age every year like everyone else. The interesting question is when: in the three years without a 29 February, do you turn a year older on 28 February or on 1 March? The answer depends on who you ask.

Why 29 February exists

The Earth takes about 365.2422 days to go round the Sun. A calendar of 365 days drifts against the seasons by almost a quarter of a day each year, so the Julian calendar added a leap day every fourth year. That over-corrected slightly, so the Gregorian calendar, introduced in 1582, drops three leap days every 400 years:

• A year is a leap year if it divides by 4,
• except century years, which are only leap years if they divide by 400.

So 2000 was a leap year, but 1900 wasn’t, and 2100 won’t be. The next leap days are in 2028, 2032 and 2036.

How rare is a leap day birthday?

In a four-year cycle there are 1,461 days and only one of them is 29 February, so if births were spread evenly about 1 in 1,461 people would be born on it. Over the full 400-year Gregorian cycle, which skips three leap days, it’s 97 days in 146,097, about 1 in 1,506. With roughly 8 billion people on Earth, that works out at around 5 million people born on 29 February. People born on that day are often called “leaplings” or “leap day babies”.

When do you turn a year older?

There are two conventions for the years without a 29 February:

28 February treats your birthday as the last day of February, since your birthday month has no 29th. This is the same rule calendar software uses whenever a date doesn’t exist in the target month: add one month to 31 January and you get 28 February. Most programming libraries, including the new Temporal date API for JavaScript, behave this way by default.

1 March treats your birthday as the day after 28 February, on the reasoning that a full year hasn’t passed until 28 February is over. Spreadsheets lean this way: Excel’s DATEDIF function only counts a complete year for a 29 February birth once 1 March arrives.

On 28 February 2025, someone born on 29 February 2004 is 21 years old under the first convention, and 20 years, 11 months and 30 days under the second. That one-day difference rarely matters, until it does.

What the law says

There’s no worldwide rule. Some legal systems spell out which day a person born on 29 February reaches a new age in a common year, and they don’t all choose the same day. Many say nothing at all, leaving organisations to set their own policies.

If the exact day matters (turning 18 for a contract, 16 or 17 for a driving licence, or the legal age to buy alcohol), check with the organisation or authority involved. Ask which date they treat as your birthday in a common year, and don’t assume they use the same rule as your friends, your bank or your phone.

Celebrating

Many leaplings celebrate on 28 February, keeping the party in their birth month. Others choose 1 March, and some switch between the two depending on the weekend. The real 29 February, every fourth year, usually gets the biggest celebration, and the joke never gets old: someone turning 40 is celebrating only their 10th true birthday.

How our calculators handle it

Our age calculator and birthday countdown count 28 February as the birthday in common years by default, consistent with the way they handle every other date that doesn’t exist in a shorter month. When you enter a 29 February date of birth, an option appears so you can switch to 1 March. The setting is saved in the page address, so a shared result keeps it. For the full set of rules, see our methodology.