Dates
How exact age is calculated, including 29 February birthdays
How to work out someone’s exact age in years, months and days, why the day before a birthday is the tricky case, and what happens to people born on 29 February.
The basic rule
Your age in completed years goes up by one on your birthday. So the calculation is simple: subtract the birth year from the current year, then subtract one more if this year’s birthday hasn’t happened yet.
Example: someone born on 2 November 2000 is 25 on 1 November 2026, even though 2026 − 2000 = 26. They turn 26 the next day. Forgetting that last check is the most common mistake in spreadsheets and hand-written forms.
Years, months and days
For an exact age, count whole years first, then whole months from the last birthday, then the remaining days. Born 2 November 2000, on 1 November 2026: 25 years, 11 months and 30 days.
Months have different lengths, so “one month after 31 January” has no perfect answer. Date, Age & D-day Calculator follows the common convention of counting calendar months and then the remaining days, which matches how most people and most official forms count. It also shows the total number of days lived: 9,495 in this example.
Born on 29 February
In three years out of four there is no 29 February, so when does someone born on that day become a year older? Rules differ:
- Many legal systems, including England and Wales, treat 1 March as the birthday in common years for legal age.
- Others, such as New Zealand, use 28 February.
- Many families simply celebrate on 28 February.
Our calculator uses 1 March in common years, so a person born on 29 February 2004 turns 22 on 1 March 2026. If an exact legal age matters, for a contract, a licence or a benefit, check the rule in the relevant country.
Other ways of counting age
Not every system counts completed years. In South Korea, people traditionally also used a “counting age”, in which everyone is 1 at birth and gets a year older on New Year’s Day, and some laws still use “year age” (current year minus birth year). Since 28 June 2023, Korean law uses the international age by default. The Korean version of the calculator shows all three for comparison.
Quick ways to get it right
- Use full dates, not just years, whenever an exact age matters.
- If you calculate in a spreadsheet, check a case where the birthday is tomorrow.
- For “days until” questions, such as an anniversary or a retirement date, use the countdown in Date, Age & D-day Calculator: it also handles leap years and lets you decide whether the first day counts as day 1.