About the age calculation
Age is counted in whole calendar years, then whole months, then remaining days — not by dividing total days by 365.25. The distinction matters at month ends: someone born on 31 January is one month old on 28 February, because there is no 31st to reach.
Formula
Count whole months between the two dates.
If the day-of-month has not been reached, that month is not complete — subtract one.
Years = months ÷ 12, remainder months.
Days = days from the last monthly anniversary to today.Worked example
Born 15 January 2000, today 20 January 2026: 312 whole months have passed, which is 26 years and 0 months, plus 5 days since the anniversary on the 15th.
Frequently asked questions
Why not just divide the total days by 365?
Because leap years make that drift. Over 26 years the error accumulates to more than a week, and the result cannot be expressed as the years-months-days form people actually use.
What happens if I was born on 29 February?
In non-leap years the anniversary is treated as 28 February, so age still increments once a year. This calculator clamps to the last valid day of the month rather than rolling into March.
Is this age used for legal purposes?
It follows the common convention of completed years, which is what most jurisdictions use. A few legal systems count age differently — East Asian age reckoning traditionally counts from one at birth — so check locally if it matters.
Sources
- Gregorian calendar arithmetic with month-end clamping