Leap day birthdays explained

Born on a day that does not always exist

Most people celebrate their birthday every year without a second thought. For people born on 29 February, sometimes called "leaplings" or "leapers", the date appears on the calendar only once every four years or so. That raises some fun and practical questions: how old are they, when do they celebrate, and what do forms and laws say? This guide explains why leap days exist and how birthdays on 29 February are handled.

Why 29 February exists

The Earth takes about 365.2422 days to orbit the Sun, not a neat 365. If the calendar always had 365 days, it would slowly slip out of step with the seasons, losing about a quarter of a day each year. After a few centuries, spring would arrive in the wrong month.

The Julian calendar, introduced by Julius Caesar in 45 BCE, solved this by adding an extra day every four years, giving an average year of 365.25 days. That was slightly too long, and the calendar drifted by roughly a day every 128 years. In 1582, Pope Gregory XIII introduced the Gregorian calendar, which refined the rule so that the average year is 365.2425 days, close enough to the real solar year to stay accurate for thousands of years.

Which years are leap years?

The Gregorian rule has three steps:

1. A year divisible by 4 is a leap year.
2. But a year divisible by 100 is not a leap year.
3. Unless it is also divisible by 400, in which case it is a leap year.

Over a 400-year cycle, this gives 97 leap years rather than 100. You can check any year with the day of the week calculator, which shows whether a date's year is a leap year.

How rare is a leap day birthday?

A four-year cycle has 1,461 days (three years of 365 days plus one of 366), and only one of them is 29 February. So the chance that a random person was born on a leap day is about 1 in 1,461. That is why leaplings often find that being asked "when is your birthday?" leads to a conversation.

How old is a leapling?

Age is about the time that has passed since birth, so a leapling's age is calculated just like anyone else's: by completed years. Someone born on 29 February 2000 is 26 years old in 2026, because 26 years have passed since birth. The only unusual thing is the number of actual birthdays on the calendar.

When is the birthday in a non-leap year?

In years without 29 February, people usually celebrate on either 28 February or 1 March. Neither is more correct in everyday life. For legal and administrative purposes, the answer depends on the jurisdiction and sometimes on the purpose. It is commonly reported that some places treat 1 March as the legal birthday and others treat 28 February, so for anything official, check the rule that applies to you.

Some traditions also attach to the day itself. In Irish folklore, 29 February is the day on which women may propose marriage to men.

How ChronoAge handles leap day birthdays

The calculators on this site treat 1 March as the birthday in years that are not leap years. In leap years, the birthday is 29 February.

If your own rules use 28 February instead, you can use the Age on a specific date option and enter 28 February as the date to see the result on that day.

Key takeaways

• Leap days keep the calendar in step with the seasons.
• A year is a leap year if it is divisible by 4, except century years not divisible by 400.
• About 1 in 1,461 people is born on 29 February.
• A leapling's age is counted in completed years, like anyone else's.
• In non-leap years, the birthday is treated as 28 February or 1 March depending on the rule being used.

This article is for general information. For legal or official purposes, rely on the guidance of the relevant authority.