Age is not simply the difference in years between two dates — it depends on whether the birthday has already passed this year, and on how many days each intervening month held. This calculator does the borrow-and-carry arithmetic properly and also reports total days lived and the countdown to the next birthday.
By subtracting day from day, month from month, and year from year, then borrowing where a component goes negative — borrowing the actual length of the preceding month, not a flat 30 days.
They are handled exactly, because the calculation works on real calendar dates. Someone born on 29 February simply has fewer literal birthdays; total days lived remains correct.
Yes. Change the "as of" date to work out how old someone was at a wedding, or will be on a given date in future.
Because of leap days. Over 40 years there are roughly ten extra days, which is enough to matter for anything that counts in days.
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