Summary
An age calculator determines the exact time elapsed between a birth date and a target date, expressed in years, months, and days. While the concept seems simple, calendar irregularities — months of different lengths, leap years, and timezone differences — make precise age calculation surprisingly nuanced. Most legal and medical systems define age as the number of complete years since birth, incrementing on each birthday.
How it works
The calculator computes age by:
- Counting complete years — the number of birthdays that have passed by the target date.
- Counting remaining months — the number of complete months after the last birthday.
- Counting remaining days — the leftover days after the last complete month.
The result is also expressed as total months, total weeks, and total days for convenience.
Leap year rules
The Gregorian calendar defines a leap year as:
- Divisible by 4 and
- Not divisible by 100 unless also divisible by 400
This means 2000 was a leap year, but 1900 was not. February 29 birthdays are handled by treating March 1 as the birthday in non-leap years.
The formulas
Where
Where
Worked examples
Age on 2025-02-20 for someone born 1990-07-15
Count complete years
= 34 years
Count remaining months after last birthday (July 15, 2024)
= 7 months (Jul 15 to Feb 15)
Count remaining days
= 5 days
Result
34 years, 7 months, and 5 days old
Leap year birthday: born Feb 29, 2000 -- age on Mar 1, 2025
Count complete years
= 25 years (birthday treated as passed on Mar 1)
Remaining months and days
= 0 months, 0 days
Result
25 years, 0 months, 0 days (just turned 25)
Practical uses
- Legal age verification — determining eligibility for driving, voting, drinking, retirement, and contracts.
- Medical milestones — pediatric growth charts and vaccination schedules rely on exact age in months and days.
- Insurance and pensions — premiums and benefit amounts often depend on exact age at a specific date.
- Astrology and genealogy — precise birth-to-date calculations for charts and family records.
Assumptions & limitations
- Gregorian calendar only — the calculator assumes the Gregorian calendar throughout. Dates before the Gregorian adoption (1582 in most of Europe, later elsewhere) may not be historically accurate.
- No timezone handling — the calculation uses calendar dates without considering timezones or the exact time of birth.
- Feb 29 birthdays — in non-leap years, conventions vary by jurisdiction. Some legal systems treat March 1 as the birthday; others use February 28. This calculator uses March 1.
- Future dates — entering a birth date in the future produces a negative age, which the calculator displays as zero.