Calendar-based calculation
The earlier date is used as the starting point. The calculator first adds as many complete calendar years as possible without passing the later date. It then adds complete calendar months and counts the remaining days.
Why we do not divide by 365
A calendar year may contain 365 or 366 days, while a month may contain 28, 29, 30 or 31 days. Dividing a total by a fixed average can estimate a duration, but it cannot produce a dependable calendar breakdown.
Month-end behaviour
When a target month does not contain the original day number, the calculation uses the final valid day of that month. This rule is applied consistently before the remaining days are counted.
Test coverage
- Dates in either order
- Identical dates
- February 29 and non-leap years
- January 31 and shorter following months
- Consecutive dates and year boundaries
- Rejection of future dates of birth
The tool runs locally in the browser. Entered dates are not transmitted to a calculation server.