How the calculation works
The calculator validates both entries as Gregorian calendar dates, then finds complete years from the date of birth. It finds complete months after that anniversary and counts the remaining days. It also reports the total number of elapsed calendar days separately.
Month-end anniversaries are clamped to the last valid day of the target month. A February 29 birth date therefore reaches a one-year calendar age on February 28 in a non-leap year under this explicit convention.
Conventions and boundaries
- Birth date: must be on or before the as-of date.
- Dates: use YYYY-MM-DD from 0001-01-01 through 9999-12-31.
- Calendar only: no time of day, timezone, daylight-saving, or leap-second adjustment is used.
- Scope: the result is a calendar duration, not a legal age or eligibility determination.
Common questions
Does this calculate age as of a past date?
Yes. Enter any valid as-of date on or after the date of birth.
How are leap-day birthdays handled?
February 29 anniversaries clamp to February 28 in a non-leap year, matching the shared Date Duration convention.
Is this a legal-age calculator?
No. Legal eligibility can depend on jurisdiction, local rules, exact time, and other facts. This tool reports only a stated calendar duration.
Version and verification
Calculation version 1.0.0; deterministic fixtures last verified 2026-08-19. The engine reuses the independently tested Date Duration calendar implementation.