How the age calculator works
Age is the elapsed time between a date of birth and a reference date, usually today. Most people want it in years, months, and days, which is a calendar count rather than a division: years advance on each birthday, months advance on each monthly anniversary of the birth day, and days fill the remainder.
years = full years since the date of birth months = full months since the last birthday days = days since the last monthly anniversary
Worked example
Someone born May 15, 1990, on September 13, 2026:
last birthday May 15, 2026 -> 36 years last monthly anniversary Aug 15, 2026 -> 3 months Aug 15 to Sep 13 -> 29 days age = 36 years, 3 months, 29 days
The totals come from the exact day count. May 15, 1990 to Sep 13, 2026 is 13,270 days (36 years x 365, plus 9 leap days, plus the 121 days from May 15 to Sep 13). That is 1,895 weeks and 5 days, 435 whole months, or 318,480 hours. The next birthday, May 15, 2027, is 244 days away.
Month-end and leap-day birthdays
Calendar months are uneven, so the monthly anniversary of a 31st can land in a month with only 30 or 28 days. This calculator uses the last day of the shorter month as the anniversary, which is how banks, registrars, and most other age calculators handle it. Someone born January 31 is exactly one month old on February 28 (or February 29 in a leap year), and one month and one day old on March 1. People born on February 29 celebrate on February 28 in common years for the purpose of this count.
Age conventions around the world
| System | Where used | How it counts |
|---|---|---|
| International (this calculator) | Nearly everywhere | Age 0 at birth, +1 on each birthday |
| East Asian traditional | Historical Korea, China; some ceremonial use | Age 1 at birth, +1 every Lunar New Year |
| Completed months | Pediatrics under age 2 | Total months, e.g. 18 months rather than 1 year 6 months |
| Decimal age | Growth charts, some research | Days lived / 365.25, e.g. 36.33 years |
Common uses
- Eligibility cutoffs: set the "age at" date to the cutoff (kindergarten enrollment, sports leagues, retirement) to see whether the threshold is met on that day.
- Milestones: the total days figure makes it easy to find your 10,000th or 20,000th day, which fall around ages 27 and 55.
- Forms and records: medical and legal forms often want completed years and months rather than a birth date.
To count days between two arbitrary dates, or to add months and years to a date, use the date calculator.