Leap Day Birthdays: When Someone Born on 29 February Turns a Year Older

Someone born on 29 February has a birthday on the calendar only in leap years. In the other three years out of four, the question is which neighbouring day stands in for it: 28 February, the last day of their birth month, or 1 March, the day after 28 February ends. Both answers are in everyday use, and where the law has an opinion, different places have chosen differently.

This guide covers how uncommon leap-day births are, the reasoning behind each convention, which jurisdictions have written a rule down, how old a leap-day person is on the days either side of the missing birthday, and a few practical consequences for forms, age limits and celebrations.

How rare is a 29 February birthday?

The Gregorian calendar repeats every 400 years. One full cycle holds 146,097 days, of which 97 are 29 February, because century years are leap years only when they divide by 400. If births were spread evenly over the calendar, about 1 in 1,506 people would be born on a leap day, or roughly 0.066 per cent. Within a single four-year stretch that contains a leap day, the share is 1 in 1,461.

Real births are not spread evenly. Birth rates vary with the season and with the weekday, partly because planned deliveries are less common at weekends, so the true share in any country can drift a little either side of that estimate. It is a useful order of magnitude, not a count.

Because 1,461 days is 208 weeks and 5 days, a leap day falls five weekdays later than the one four years before it, or equivalently two weekdays earlier. That holds only while leap years are four years apart. Across a skipped century year the gap is eight years, or 2,921 days, so 29 February 2104, a Friday, falls 2 weekdays after 29 February 2096, a Wednesday. Over the next few leap years that gives:

28 February or 1 March?

The two conventions come from two reasonable ways of reading “one year later”.

• 1 March treats a year as complete only when the date after 28 February arrives. A person born on 29 February was born after the whole of 28 February had passed, so in a year with no 29th, the first moment that is clearly past the anniversary is the start of 1 March.
• 28 February keeps the birthday inside the birth month. Many date systems handle a missing day by falling back to the last day of the month, the same way a monthly payment due on the 31st is taken on the 30th in shorter months.

Where lawmakers or courts have addressed the question, they have not agreed:

Many other legal systems contain no specific rule, and the answer then depends on how a particular authority or court reads the general law. Rules can also differ within one country: the New Zealand driver licensing rule names 28 February for its own purposes, while the country’s general law on attaining an age refers only to the start of the anniversary.

Age either side of the missing day

Here is the age of someone born on Friday, February 29, 2008 on the days around the end of February, under each convention. The Handy Bento age calculator follows the 1 March column.

The two rules only disagree on 28 February in common years, a single day each time. In a leap year they agree again, because the real birthday exists. That single day is exactly the one that matters when an 18th or 21st birthday falls in a common year, as it does for this person in 2026: under one rule they become an adult on a Saturday, under the other on a Sunday.

Counting real birthdays

People born on a leap day like to joke that they are a quarter of their age. The joke is nearly right, but the century rule makes it uneven. From 1904 to 2096 every fourth year is a leap year, yet 1900 was not and 2100 will not be, so anyone whose first four years include one of those century years waits eight years for the first real one.

Someone born in 1896 had to reach the age of 8 before seeing a 29 February birthday, in 1904. The next people to share that experience will be those born in 2096, whose first real birthday is in 2104. For births from 1904 to 2056 the count by age 40 is always 10; anyone whose first forty years include 1900 or 2100 gets only 9.

Practical consequences

Forms and records

A birth certificate, passport or identity card for a leap-day person shows 29 February, and that is the date to enter on every form, whatever the current year. Do not substitute 28 February or 1 March on paperwork: a mismatch with other records is far more likely to cause trouble than the missing date itself.

Age limits on a specific day

Age checks at a shop counter or a door are usually made against the printed birth date and the rules the business follows, so someone turning 18 on a common-year 28 February may be refused where the 1 March reading applies. Planning a first legal purchase or a landmark trip for 1 March avoids the argument in most places.

Deadlines described as an anniversary

Contracts, warranties and subscriptions that start on 29 February run into the same question when they renew yearly. Well-drafted terms say what happens; otherwise the provider’s systems decide, and they may pick either date. It is worth checking the renewal date shown in your account rather than assuming.

When to celebrate

There is no rule for parties. Families pick 28 February to stay in the birth month, 1 March to wait until the year has truly turned, or both. Some make the real birthday in a leap year the bigger celebration, since it comes round only once every four years.

Checking an exact age

To see a leap-day person’s exact age on any date, enter 29 February as the date of birth in the age calculator and set the date you care about. On 28 February of a common year it shows 11 months and 30 days past the last birthday, and on 1 March it shows the new age with a note explaining the rule. If you need the 28 February reading instead, count the new age one day earlier than the calculator does.