How this calculator works
Completed years count how many birthdays have passed on the calendar. The birthday is rebuilt in the as-of year; if it has not yet occurred, one is subtracted. Because that leaves out any months since the last birthday, the page also reports whole months and total days, so the same age can be read at three levels of precision.
Formula
years = as_of.year - birth.year, minus one when the birthday has not yet been reached in the as-of year. A February 29 birthday is treated as February 28 in a common year. Whole months and total days are taken from the same interval.
Worked example
Born 1990-05-20 and measured on 2026-11-01, you are 36 completed years old, which is also 437 whole months or 13,314 days. The next birthday on 2027-05-20 will take the completed-years figure to 37.
Advertisement
Frequently asked questions
Is this legal age?
The arithmetic gives the completed-years figure most statutes use, but legal age depends on the jurisdiction and on whether a February 29 birthday is treated as February 28 or March 1. Check the applicable law before relying on the result for a deadline.
Why is the month count useful if I asked for years?
Two people with the same completed years can be almost a year apart. The whole-month figure shows that the 36-year-olds in the example are really 437 months old, which ranks them more precisely.
How are leap-day birthdays handled?
A February 29 birthday is placed on February 28 in non-leap years, so the completed-years figure advances on that date. Some people prefer to celebrate on March 1; the year count would then differ on February 28 and March 1 of common years.