Time & date guide
How to Calculate Your Age Exactly
Age sounds trivial until you try to state it exactly: years, months, and days don't divide evenly into each other, and February 29 exists only some years. Here's how exact age is actually computed.
Last reviewed 2026-08-01
Calendar age: the birthday method
Years advance on your birthday, months on your monthly anniversary, and days cover the rest. Born June 15, 1990 and measuring on August 23, 2026: 36 years (birthday passed), 2 months (June 15 → August 15), 8 days (August 15 → 23).
When the day-of-month hasn't arrived yet — say measuring on August 10 — the months borrow one: the age reads 36 years, 1 month, and however many days sit between July 15 and August 10 (26 days, since July has 31).
Leap years and February 29
A leap day is added in years divisible by 4, except century years, unless divisible by 400 — so 2000 was a leap year but 1900 was not, and 2100 won't be.
People born on February 29 legally turn a year older on February 28 or March 1 in non-leap years depending on jurisdiction. For day-counting purposes it makes no difference: every day counts as one day.
Day counts vs calendar age
Total days lived is a simple subtraction of dates — every day identical. Calendar age is irregular by design because months have 28–31 days.
That's why '10,000 days old' (about 27 years and 4.5 months) is a fun milestone the calendar age hides.
Common pitfalls
- Subtracting month numbers directly without borrowing when days go negative.
- Assuming all months have 30 days — the borrowing month decides how many days to add.
- Forgetting that 'inclusive' counts add one day: from Monday to Monday is 7 days, or 8 counted inclusively.