Age Calculator

How the age calculator works

The calculator counts full years first, then the remaining months, then the leftover days — the way people naturally describe age. It accounts for different month lengths and leap years, so the result is exact rather than an average. Change the second date to measure the gap between any two dates: an anniversary, a project deadline, or how long until an event.

How exact age is worked out

Age is not simply days divided by 365. Calendar age counts completed years, then completed months, then leftover days — a “borrow” process much like long subtraction, because months have different lengths.

Leap years and the 29 February problem

A leap year occurs every year divisible by four, except centuries, unless divisible by 400 — so 2000 was a leap year and 1900 was not. That gives an average calendar year of 365.2425 days, which is why any “age in years” derived by dividing total days drifts by a day or more.

People born on 29 February have no birthday in common years. Jurisdictions differ on which date counts for legal purposes: some treat 28 February as the anniversary, others 1 March. This calculator counts a leaping birthday as reached on 1 March in non-leap years.

Other ways of counting age

• Western reckoning — you are 0 at birth and gain a year on each birthday. The default here.
• East Asian age reckoning — traditionally you are 1 at birth and everyone advances at new year, so a person can be two “years old” days after being born. Now largely ceremonial.
• Insurance “age nearest birthday” — some policies round to the closest birthday rather than the last one, which can make you a year older on paper.
• Gestational and corrected age — used for infants born prematurely, measured from the due date rather than the birth date.

Other things this tool answers

Because it measures any interval between two dates, the same calculation covers a project duration, a length of service, the gap between two historical events, a pet’s age, or how many days remain until a deadline. Enter the earlier date first and the later date second; the result is the exact span, not an estimate.