Common birthday-based age
The usual age system increases age on the birthday. Someone born on 2000-01-01 turns 26 on 2026-01-01, regardless of the number of leap days between those dates.
Total days and approximate months
Total days is a precise elapsed-time count between dates. Approximate months are less exact because months differ in length.
Common mistakes
- Using total days divided by 365 as exact age in years.
- Ignoring leap-day birthdays.
- Using local time zones when the source dates are UTC timestamps.
- Assuming every form or institution defines age the same way.
Named age measures
Age can mean completed years, total months, total days or age on a specific date. Legal, school, insurance and sports rules may define the relevant age differently.
Concrete edge cases
A person born on February 29 may have different administrative birthday handling in non-leap years. A person born late at night in one time zone may have a different local date in another time zone if timestamps are involved.
What to specify
Date of birth.
As-of date.
Calendar system.
Time zone if timestamps are used.
Whether the result needs completed years or elapsed days.
Use the calculators
FAQ
Is age in months exact?
Calendar months can be exact when expressed as years, months and days. Decimal months are approximate.
How are leap-day birthdays handled?
Rules vary by context. A generic calculator needs to show the date arithmetic and avoid legal assumptions.
Why show total days?
Total days gives an exact elapsed-day check that is independent of calendar month names.
Last reviewed: 2026-05-14.