Age Calculator
Calculate exact age in years, months, and days between two dates, plus days to the next birthday.
Inputs
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Formula
Age = (as-of date) − (date of birth), broken into years, months, and days
Calendar-aware subtraction: months and days borrow from the preceding month/year the same way you'd count on your fingers, not a flat division of total days by 365.
Worked example
Someone born on 1 January 2000, measured on 30 August 2026, is 26 years, 7 months, and 29 days old — and about 213 days from their next birthday.
Where this can give the wrong answer
- A person born on 29 February only has a real birthday every 4 years; in a non-leap year, this calculator's 'next birthday' rolls forward to 1 March rather than 28 February, since that's what constructing that calendar date actually produces.
- Total days lived is a flat day count and will differ slightly from years×365 because of leap years crossed.
- An 'as of' date before the date of birth is rejected as an error rather than returning a negative age.
FAQ
- Because leap years add an extra day roughly every 4 years. Over multiple decades those extra days add up to more than you'd expect from a naive years×365 estimate.