How is age counted across months of different lengths?

A month runs to the same day number in the next month, or to that month’s last day when it is shorter. From 31 January, one month reaches 28 February — so 31 January to 1 March is one month and one day.

When does someone born on 29 February have a birthday?

On 1 March in common years, which is the usual civil convention, and on 29 February in leap years. This calculator follows that rule for the countdown.

Why does the calculation use UTC?

So the answer does not change with the visitor’s time zone, and daylight saving cannot shift a day count by one. Dates are treated as calendar dates, not moments in time.

Time & dates

Age Calculator

Enter a date of birth to see the exact age, and how long until the next birthday.

The formula

age = whole months since birth, then the leftover days

a month ends on the same day number, or the last day of a shorter month

Worked example

Born 15 June 1990, measured on 10 August 2026

36 whole years and 1 month reach 15 July 2026
15 July to 10 August is 26 days
Age: 36 years, 1 month, 26 days

Where this goes wrong

Working age out from years alone

Someone born on 31 December 2000 is not 25 for most of 2025. Age turns over on the birthday, not on New Year, so subtracting years overstates it for everyone who has not yet had their birthday this year — roughly half the population on any given day.

Questions

How is age counted across months of different lengths?: A month runs to the same day number in the next month, or to that month’s last day when it is shorter. From 31 January, one month reaches 28 February — so 31 January to 1 March is one month and one day.
When does someone born on 29 February have a birthday?: On 1 March in common years, which is the usual civil convention, and on 29 February in leap years. This calculator follows that rule for the countdown.
Why does the calculation use UTC?: So the answer does not change with the visitor’s time zone, and daylight saving cannot shift a day count by one. Dates are treated as calendar dates, not moments in time.