Exact Age Calendar Math: Years, Months, Days, and Leap Birthdays
Chronological age is a calendar decomposition, not an average-year estimate. This guide explains how the age calculator advances complete years, then months, then remaining days—and how that differs from a raw day count.
Date-only inputs, Gregorian calendar
The age worksheet treats birth date and as-of date as Gregorian calendar dates (ISO-style YYYY-MM-DD), not clock times. Internally, total elapsed days use a UTC day index so daylight-saving 23- and 25-hour local days do not change the span. Date formatting conventions for civil dates are discussed in RFC 3339; legal age rules still vary by jurisdiction.
Decomposition beats “days ÷ 365”
The calculator finds the greatest whole number of years that can be added without passing the as-of date, then the greatest whole number of months, then counts remaining days. Months have 28–31 days and years have 365 or 366 days, so dividing total days by 365 (or 30) cannot reproduce a calendar age label.
February 29 on this site
For deterministic arithmetic on this calculator, a February 29 anniversary is clamped to February 28 in a non-leap year. That rule is stated on the age calculator page so results stay reproducible; it is not a claim about every jurisdiction’s recognition of age.
Related date tools
Need age as Y/M/D?
Use the age calculator with an explicit as-of date.
Need a raw span?
Use days between dates and match its inclusive/exclusive rule to your contract language.
Need business days?
Use working days with weekends and holiday lists you supply.
Need a date offset?
Use the date calculator to add or subtract days from a start date.