Age calculator

Age is counted the way a birthday is counted, not by dividing days. Whole years are added to the date of birth while they still fit, then whole months, then the days left over. So someone born on 31 March is one month old on 30 April and 1 month 1 day old on 1 May: a month that has no 31st ends on its last day. Someone born on 29 February has a birthday on 28 February in common years, which is the rule most registries use; February 29 itself returns every fourth year, and the next-birthday line names the real date it falls on.

Both dates are turned into day counts on the proleptic Gregorian calendar with integer arithmetic, so every year from 1 to 9999 counts alike, leap years included. Clock times, birth hours and time zones are not part of it: a day is a day wherever it is spent, and a page that asked for your hour of birth could not tell you anything more exact. In days is the plain difference between the two dates; in weeks is that divided by seven with the remainder; in months counts whole months the same way the years do. A date of birth after the second date is refused rather than shown as a negative age — swap the two fields to count towards a birth.

Nothing is uploaded and nothing is remembered: there is no server in this file, and the dates you type are gone when the tab closes. The Dark button remembers its own setting in this browser's local storage, and nothing else is stored.