Age Calculator

Age looks like subtraction and is not, because the calendar is irregular. From June 15, 1990 to October 7, 2026 is 13,263 days, and that number is correct and almost never what anybody wants. The useful answer is 36 years, 3 months and 22 days, and you cannot get it from the day count without consulting the actual calendar. The months in between were 28, 30 and 31 days long in some order that depends entirely on which months they were.

The right method counts coarse units first. Take whole years from the birth date, then whole months from there, then measure the leftover days. That ordering is what keeps the day component from going negative. The naive approach (subtract the years, subtract the months, subtract the days, then borrow when something goes negative) fails on exactly the cases where borrowing is needed. From January 31 to March 1 the day difference is -30, and February has only 28 days to lend, so the result stays negative however carefully the lending month is chosen.

Then there is the question of which convention of "age" you mean, and it varies by culture more than most people realize. The Western convention is completed years: you are 35 until the moment you have completed 36 of them. East Asian age reckoning traditionally counted a person as one at birth and added a year at each lunar new year rather than at each birthday, which could make a baby born in December "two years old" a few weeks later. South Korea used that system alongside two others until 2023, when it legally standardized on the international count. This calculator uses completed years throughout.