How to use
- Pick the birth date.
- Pick the as-of date. Use today if you want age as of today, or any later date.
- Press Calculate. The hero is completed years. Months and days are the remainder.
- Total days counts midnights between the dates and will not match years × 365.
How it's calculated
Subtract the year, month, and day. If the day of the month has not been reached, borrow the length of the previous month and reduce the month count. If the month has not been reached, borrow 12 months and reduce the year count.
Total days is the UTC date difference, so a time zone does not add or remove a day. February 29 is a real day only in leap years. If you enter it in a non-leap year, the date field will not accept it.
Worked example
Someone born on 15 March 1990, as of 26 September 2026, is 36 years, 6 months, and 11 days old. September is six months after March, and 26 − 15 is 11 days. The total-day count for those two dates is 13,344 days, which is not 36 × 365.
Assumptions
Dates only, no time of day. The as-of date must be on or after the birth date. The page does not adjust for the exact hour of birth.
FAQ
Why are total days not years times 365?
Leap days and the varying length of months sit inside the span. The year-month-day display follows the calendar. The day total counts every date.
Can I measure age in months only?
Completed months are years × 12 + the leftover months. The leftover days are not a full extra month.
Does the as-of date have to be today?
No. Any date on or after the birth date works, which is how you check age on a future birthday or a past date.
What about a birthday on February 29?
Enter February 29 only in a leap year. The next anniversary in a non-leap year is a date you choose, usually February 28 or March 1. The page will not invent that anniversary.