Ask someone their age and they’ll say a single number. But calculating that number correctly — and getting the months and days right too — trips up a lot of spreadsheet formulas.
Why “current year minus birth year” is wrong
If you were born on December 20, 2000, then on December 1, 2026, a naive 2026 − 2000 = 26 says you’re 26.
You’re not — you’re still 25, because your birthday hasn’t happened yet this year. The subtraction only
becomes correct once the birth month and day have both passed.
The correct method
- Start with
years = currentYear − birthYear. - If the current month/day is before the birth month/day, subtract 1 from
years. - For the leftover months and days, walk forward from the last birthday to today, borrowing a month’s worth of days when the day-of-month is negative (this is the same borrowing logic used in manual subtraction).
This is exactly what a calendar app does, and it’s why age calculators can’t just divide total days by 365.25 — months have 28 to 31 days, and calendar years have 365 or 366, so any fixed-average approach drifts off by a day or two here and there.
Total days lived vs. years/months/days
Both numbers are useful for different things. “Total days lived” is a straight subtraction of two dates (handy for milestone dates like your 10,000th day). “Years, months, days” is the calendar-aware version people actually use in conversation — and the one prone to off-by-one errors if you compute it by hand.
Try it yourself
The Age & Date Calculator computes both, plus the days remaining until your next birthday, entirely in your browser.