Age Calculator

Counting years, months and days

Age in years is the number of complete years elapsed, which means someone one day before their birthday is still the previous age.

Calculating years, months and days requires borrowing when the current day is earlier in the month than the birth day, and the borrow uses the length of the preceding month, which varies.

That is why age calculations disagree at the margins. Different implementations borrow differently, and a person born on the 31st has no anniversary in months with 30 days, which requires a convention rather than an answer.

Leap years and 29 February

A leap year occurs every four years, except century years, except century years divisible by 400. So 1900 was not a leap year and 2000 was.

Someone born on 29 February has a birthday every four years. Legally, most jurisdictions treat their birthday as 1 March in non-leap years for the purpose of reaching an age, though some use 28 February.

For age calculation the practical effect is small and it is a genuine ambiguity, which is why implementations differ and why a leap-day birthday occasionally produces an off-by-one result.

Age reckoning elsewhere

East Asian age reckoning traditionally counted a person as one year old at birth and added a year at each new year rather than at each birthday, which could make someone born in December two years old within weeks.

South Korea used this system in daily life until 2023, when legislation standardised on international age for legal and official purposes.

The systems still surface in conversation and in older documents, and it is worth knowing that a stated age in some contexts may not mean years since birth.

Where the exact figure matters

Legal thresholds are the obvious case: voting, driving, drinking, working, retirement and pension ages all attach to a specific date.

Medical dosing in paediatrics uses exact age, and in neonates it is measured in days and corrected for prematurity, which is a separate calculation entirely.

School year placement uses a cut-off date that varies by country and sometimes by region, and a child born days either side of it can start a full year apart. That difference has measurable effects on attainment that persist for years, which is a well-studied phenomenon.

Time zones and the date of birth

A birth is recorded in the local time zone where it occurred, which means a birth at 11pm in one zone was 4am the next day in another.

For anyone born abroad or crossing time zones, the recorded date is what documents use, regardless of what the local time was where their family lived.

This matters mainly for astrological calculations, which need a precise local time, and occasionally for legal thresholds falling on a particular day. For everyday purposes the recorded date is the answer and the underlying instant is not worth pursuing.

Age in days is worth mentioning because it is the version that has genuine uses and gets asked for oddly rarely. Neonatal care works in days and hours rather than years. Some legal and immigration processes count days of residence. And people occasionally want to know the date of a round number of days lived, which is a pleasant enough reason on its own. Days are also the only unambiguous unit here, since they need no convention about month lengths or leap years, which is why software stores dates as day counts internally and converts to calendar terms only for display.