Roughly 5 million people worldwide share a birthday that only exists once every four years. If you’re one of them — or you’re just curious how age calculators handle it — the answer turns out to be more legally interesting than you’d expect.

Why February 29 exists at all

A calendar year of 365 days doesn’t perfectly match the time Earth actually takes to orbit the sun — about 365.2422 days. Without adding an extra day roughly every four years, the calendar would slowly drift out of sync with the seasons. The modern rule (part of the Gregorian calendar) is: a year is a leap year if it’s divisible by 4, except century years, which must also be divisible by 400. That’s why 1900 wasn’t a leap year, but 2000 was.

So which day do “leaplings” actually age on?

In the three out of four years without a February 29, the law still has to decide when someone born on that date legally turns a year older. Different countries have landed on different answers:

  • England and Wales: Under the Interpretation Act 1978, a person born on February 29 legally turns a year older on March 1 in non-leap years.
  • Hong Kong: The Age of Majority (Related Provisions) Ordinance instead uses February 28 as the legal birthday.
  • New Zealand: The Births, Deaths, Marriages, and Relationships Registration Act also defaults to February 28.
  • United States: There’s no single federal rule — it varies by state, though most default to March 1. Individual agencies like the DMV or Social Security Administration may apply their own interpretation on top of that.

A useful shortcut for figuring out which convention a jurisdiction likely follows: if a state or country’s legal system excludes the day of birth when computing age, March 1 tends to be the operative date. If it includes the day of birth, February 28 is used instead.

It’s not just a trivia question — it affects real legal rights

This isn’t merely academic. The date determines when a leapling can legally drive, vote, drink, or collect retirement benefits in a given non-leap year. Courts have occasionally had to weigh in directly — for instance, criminal sentencing law has had to clarify whether a “one month” sentence is a fixed 30 days, regardless of whether it spans a leap day.

How age calculators typically handle it

In software, the near-universal convention is to treat a February 29 birth date as February 28 for the purposes of year-boundary calculations in non-leap years — this is how major programming libraries (including Python’s dateutil and Java’s LocalDate) handle it by default. Practically speaking, this calculator follows the same logic: your age updates at the start of March 1 in common years, consistent with how most English-speaking legal systems (UK, Hong Kong, New Zealand-adjacent conventions) treat the date.

Japan’s Act on Counting of Age (1902) has a genuinely different quirk — a person legally gains a year of age at the start of the day before their birthday, not on the birthday itself. This is why Japan’s school-year cutoff is April 1 rather than April 2: a child born on April 2 legally turns 6 on April 1 of the relevant year. It’s a good reminder that “how old someone is” is ultimately a legal convention, not just a math problem.

Sources: Interpretation Act 1978 (England and Wales); Age of Majority (Related Provisions) Ordinance (Hong Kong); Births, Deaths, Marriages, and Relationships Registration Act (New Zealand); Act on Counting of Age, 1902 (Japan).