Formula and method
Calendar age is found by subtracting the birth date from the target date, borrowing days and months across calendar boundaries.
Worked example
A person born 10 January 2000 is 25 years, 2 months and 5 days old on 15 March 2025.
Assumptions
- The birth date and target date use the local calendar date.
- A future birth date is invalid.
Reference sources
Reviewed and updated 2026-07-14