Born on 29 February? How an age calculator counts your birthday

In a non-leap year there is no 29 February, so a leap-day birthday has to be placed on another date. The age calculator on this site uses 1 March in those years, which means the age goes up on 1 March. Other people, and some local rules, use 28 February instead. Both conventions are in use, so the right answer depends on the rule that applies to you, not on arithmetic alone.

Why 29 February exists

The calendar has a leap day because a solar year is about 365.2422 days long, not 365. A year is a leap year if its number is divisible by 4, with one exception: century years, such as 1900, are leap years only if they are divisible by 400. So 2000 was a leap year, 1900 was not, and 2100 will not be. The next leap year after 2026 is 2028. The rule above is the Gregorian one, which most countries use today. Records written before a country adopted that calendar can use different dates, so check old documents against the calendar of their time.

In short, a year is a leap year when it is divisible by 4 and not by 100, or when it is divisible by 400. The age calculator on this site applies exactly that test. The gap between leap years is normally four years, but across 2100 it is eight, from 2096 to 2104. A 29 February date therefore falls every four years in most stretches, and less often across a century year that is not a leap year.

What the age calculator does with a leap-day birthday

The calculator counts full years by checking the anniversary of the birth date in each year. For a 29 February birth, the anniversary in a non-leap year is 1 March. Take someone born on 29 February 2000:

• On 28 February 2027, the calculator shows 26 years old.
• On 1 March 2027, it shows 27 years old.

The result also gives the next birthday and the days until it. On 4 November 2026 the next birthday for that person is 1 March 2027, which is 117 days away, and the result reads "when you turn 27". In a leap year such as 2028, the anniversary is 29 February itself, and the calculator uses that date. A note under the result appears whenever the date of birth is 29 February, so you can see which convention is being used.

Dates are counted as calendar days, so the time zone you are in does not change the answer. The only thing that changes it is the date you enter. The weeks and months figures come from the same two dates.

Why local rules differ

Some rules and organisations need an age or a birthday for a leap-day person in a non-leap year. Which date they use is set by the rule that applies to them, and it is not something arithmetic can settle. Some use 28 February, some use 1 March, and some treat the birthday as falling on either. Some people celebrate on one date and count their age from the other.

The rules are set by each country or organisation, and they can differ for different purposes, such as an age limit or a pension start date. This site does not give legal advice. If a deadline, an age limit or a benefit depends on your birthday, check the rule that applies to your case with the body that administers it.

Adding years with the date calculator

The date calculator follows a different rule from the age calculator, and it is worth knowing the difference. Its note says that when adding months or years keeps the same day number but the target month is shorter, the result is the last day of that month. Adding one year to 29 February 2028 therefore gives 28 February 2029, because 2029 is not a leap year. Adding one month to 31 January 2025 gives 28 February 2025.

The two tools answer different questions. The age calculator asks when a birthday falls, and uses 1 March. The date calculator asks what plain date arithmetic gives, and uses the last day of February. For counting days, the date calculator gives an exact total. From 4 November 2026 to 1 March 2027 is 117 days. The guide to counting days between dates shows more examples.

Edge cases

• Before the anniversary. On 28 February in a non-leap year, a leap-day person has not yet reached the next birthday, so the calculator shows the lower age. The age goes up on 1 March, not on the day before.
• Total days. The Days figure is a plain count of calendar days from the birth date to the date you choose. It counts 29 February in every leap year without a special case.
• Choosing a convention. If a form asks for your birthday, enter the date you were born. If you are choosing how to count your own age, pick one convention and use it every time, so that the figures stay consistent.
• Entering the year. A birth year after today is refused. A mistyped year is the usual cause, so check the year first.
• Recording the result. If an age matters for a form or a record, write down the date you calculated it on, so the figure can be checked later.