Subtracting two birth years gives a rough age, but an exact age in years, months, and days takes a little more care — mostly because months have different lengths and leap years exist.
The method
Start from the birth date and count up:
- Whole years between the birth date and the target date.
- Remaining whole months after those years.
- Leftover days after those months.
When the day-of-month of the target is earlier than the birth day, you "borrow" a month and add the number of days in the previous month — which is why month length matters. Leap years are handled automatically when you work with real calendar dates rather than fixed 30- or 365-day approximations.
The Age Calculator does this for any two dates and also reports the total in months, weeks, and days.
Why not just use 365.25 days?
Dividing total days by 365.25 gives a decimal age that's close but not exact, and it can't express "3 years, 2 months, 14 days." For birthdays, anniversaries, eligibility dates, and legal ages, the calendar-accurate method is the one you want.