How age is actually calculated
Age looks simple — until you try to subtract two dates by hand. Here's the calendar arithmetic your age calculator runs for you.
4 min readReviewed May 1, 2026
Quick answer
Your age is the number of completed years between your birth date and a reference date, plus the leftover months and days. The trick is doing the borrowing right when months don't line up.
The exact calendar method
Given a birth date and a reference date, work top to bottom:
1. 1. Subtract years
Reference year minus birth year. If the reference month/day hasn't reached the birth month/day yet, subtract one.
2. 2. Subtract months
Reference month minus birth month. If negative, add 12 and borrow a year from step 1.
3. 3. Subtract days
Reference day minus birth day. If negative, borrow the days in the previous month and subtract a month from step 2.
Common questions
- Is your age the same on the day before your birthday in every culture?
- Mostly yes in the West. East Asian age reckoning historically counted you as 1 at birth and added a year at New Year — different system, different number.
- What about time zones?
- Legal age usually follows the local calendar where you were born, not where you are now. The calculator uses dates only, no time zones.