Calculate age on a chosen date
- Enter the birth date using the date picker.
- Choose today or another as-of date on or after birth.
- Calculate calendar age, total days and the next birthday; copy or download the result.
Calendar convention and example
We count the greatest completed number of calendar months from the original birth date. If its day is missing in a target month, we use that month’s last day. Completed months are split into years and months, then remaining calendar days are counted from that anchor. A birth date of 2000-01-01 evaluated on 2026-10-09 gives 26 years, 9 months and 8 days.
For 2023-01-31 to 2023-02-28 the result is 1 month and 0 days. For a February 29 birth, the anniversary is February 28 in non-leap years under this convention. At the birthday itself, the countdown is 0. Other tools or official rules may choose another convention.
We apply the Gregorian leap-year rule explained by the U.S. Naval Observatory: divisible by 4, except century years must also be divisible by 400. Thus 1900 is not a leap year and 2000 is.
Limits and privacy
Dates 0001-01-01 through 9999-12-31 use Gregorian rules consistently, including before historical calendar adoption. These are date-only calculations without birth times, time zones, daylight-saving changes or inclusive end-date counting. Total days count elapsed calendar boundaries: equal dates give 0. The calculator does not determine legal age, admission eligibility, identity or medical age.
Birthday output is unavailable if the next anniversary falls after year 9999. Inputs stay in browser memory without uploads or saved birth dates. CSV uses ISO YYYY-MM-DD dates with the stated month-end clamp convention. Reset restores the illustrative birth date and the local date captured when the page opened, and clears results. Undo action restores both dates before Today, the leap-day example or Reset. Calculate again after restoring; editing a date discards the previous Undo step.
Why not divide days by 365?
Years include leap days and months have different lengths. Calendar age follows actual month and year boundaries rather than an average-length year.
Does my time zone change the calculated age?
It can change the initial local today. Once you choose the two dates, the calculation uses those calendar dates consistently without time-zone offsets.