How it works
We subtract the calendar year, month, and day separately, borrowing from the previous month as needed (like long subtraction with calendars). Total days are the millisecond difference divided by 86,400,000.
FAQ
Why does the day count look "off" by one or two? We use calendar arithmetic that respects each month's actual length and leap years. Other calculators that simply divide milliseconds by 86,400,000 can drift by a day or two over decades because they ignore daylight-saving and the variable length of months.
What about "age on a specific date" — what is it for? Useful for school enrollment ("how old will my child be on the first day of class?"), insurance ("age at policy start date"), or just curiosity ("how old will I be on the next solar eclipse?").
Is anything stored? No. The birthdate stays in your browser tab. Closing the tab loses it; there is no signup.