Leap Day Birthdays: How to Count Your Age

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Leap Day Birthdays: How Do You Count Age on 29 February?

If you were born on 29 February, your birthday officially exists only in leap years. So how old are you, and when do you celebrate in the other three years? This article explains why the leap day exists, the exact leap year rule, the common ways leaplings handle their birthday and what an age app has to decide. Why there is a leap day at all A year is the time the Earth takes to go once around the Sun, and that is about 365.24 days, not a neat 365. A calendar with exactly 365 days drifts a little every year, and after a few decades the seasons would no longer line up with the dates. To fix this, an extra day is added to February in most years that are divisible by four. That extra day is 29 February, the leap day. Adding a whole day every four years slightly overshoots, which is why the rule has exceptions. The leap year rule: 4, 100 and 400 In the calendar used almost everywhere today, a year is a leap year when it can be divided by 4. There is one exception: years divisible by 100 are not leap years, unless they are also divisible by 400. Put together: 2024 is a leap year, because 2024 divides by 4 and not by 100. 1900 was not a leap year, because it divides by 100 but not by 400. 2000 was a leap year, because it divides by 400. 2100 will not be a leap year, for the same reason as 1900. Over 400 years this gives 97 leap years instead of 100, so the average year is 365.2425 days, very close to the real length. The calendar with this rule was introduced by Pope Gregory XIII in 1582, which is why it is called the Gregorian calendar. Who are the leaplings? People born on 29 February are sometimes called leaplings or leapers. There are 1,461 days in a full four-year cycle (three years of 365 days and one of 366), and only one of them is 29 February. So a random birth has roughly a one in 1,461 chance of landing on that day. Their actual date of birth comes around only every four years, usually. There is a longer gap at century years that are not leap years: someone born on 29 February 2096 would wait until 2104 for the next one, eight years later. How old is a leapling? Age in complete years is not complicated: someone born on 29 February 2004 is 20 years old in 2024 and keeps counting from there, one year at a time, like everyone else. It is a joke that they are only five, because by 2024 they have had only five real 29 February birthdays (2008, 2012, 2016, 2020 and 2024), but the real age is counted in years elapsed. The only open question is the day in a non-leap year when the new age starts. Does a leapling become a year older on 28 February or on 1 March? Both are used in practice. 28 February or 1 March? There is no single answer, and there is no need to pick one for personal life. Many leaplings celebrate on 28 February, others on 1 March, and some on both, or hold a bigger party on the real leap day when it comes around. Legal systems differ. In some places the rule for official purposes, such as a minimum age for something, is defined in law, and different countries can choose a different day. We do not know the rules for every country, so if it matters for a legal purpose, check the rule that applies where you live. How an age app has to decide A program that calculates age needs a rule for the non-leap year. The Age Calculator app treats the birthday of a leapling as 28 February in a year without 29 February. For someone born on 29 February 2004, the list of upcoming birthdays reads 28 February 2026, 28 February 2027, then 29 February 2028, then 28 February 2029 and 2030, and the count of days until the next birthday follows this list. The age in years, months and days is calculated by a date library that counts complete months from the birth date. For a person born on 29 February 2000, it shows 22 years, 11 months and 30 days on 28 February 2023, and 23 years and 1 day on 1 March 2023. So the countdown to the birthday uses 28 February, while the full age counter turns over on 1 March. These are two ordinary conven...

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