Age calculator

Exact age from a date of birth, on today or any other date, with the arithmetic shown year, month and day.

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Why "months" in an age need a rule

Years are easy: you gain one on each birthday. Months are counted from the day of the month you were born; if that day doesn't exist (born on the 31st, looking at a 30-day month), the month completes on the last day of the shorter month. The remaining days are then counted plainly. This is the same method as the ISO 8601 libraries used in banking and government software, and matches Excel's DATEDIF(birth, today, "y"), "ym" and "md" in all but a few month-end edge cases, where DATEDIF is known to misbehave.

Leap-day birthdays

Roughly 1 in 1,461 days is a 29 February, so leap-day birthdays are rare but real. For people born on 29 February, the calculator asks which date counts in common years. Some laws fix 1 March (Hong Kong's Age of Majority (Related Provisions) Ordinance does so for reaching an age), while most software, including spreadsheet date functions, falls back to 28 February. Where it matters, check the rule that applies to you.

Questions people ask

How is exact age calculated?
Count the whole years since the birth date (a year completes on the birthday), then whole months since the last birthday, then the remaining days. Someone born 15 May 1990 is 36 years, 4 months and 12 days old on 27 September 2026.
When does someone born on 29 February have a birthday in a common year?
It depends on the jurisdiction. Hong Kong law, for example, says a person born on 29 February reaches a given age on 1 March in a common year; many other systems and most software use 28 February. Choose the rule in the calculator; it only affects dates in non-leap years.
How many days old am I?
Enter your birth date: the calculator shows your age in total days and weeks as well as years, months and days.
Can I calculate age on a specific date?
Yes. Change the "age on" date, for example to check eligibility on a closing date or someone's age at a historical event.

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