Age Calculator

Working out an age in whole years is easy. Working out an exact age in years, months and days is where most calculators quietly go wrong, because months are not the same length and the arithmetic has to account for it. Someone born on 31 January is one month and one day old on 1 March in a common year, but the naive calculation produces a negative number of days and needs correcting.

This calculator handles that by stepping forward from the date of birth a whole number of years and months first, landing on the most recent monthly anniversary, and then counting the real days from there. Where that anniversary would fall on a date the month does not have, it clamps to the last day of the month — which is the same convention a calendar app uses when you add a month to 31 January.

Day counts are done in UTC rather than local time. That sounds like a technicality, but a period spanning a daylight saving change is 23 or 25 hours long, so a calculator that subtracts timestamps and divides by the length of a day can be a full day out twice a year.

You also get the total days, weeks, months and hours lived, the day of the week you were born, and how long until the next birthday. Because 29 February only comes round once every four years, a leap day birthday is treated as falling on 1 March in common years, which is the convention most legal systems use.

Set the second date to something other than today to work out how old someone was, or will be, on any particular date — useful for eligibility cut-offs, school year rules and forms.