Birthday Calculator

A birthday calculator takes your date of birth and reveals the facts hidden inside it: the day of the week you were born, your exact age in years, months, and days, the total number of days you have been alive, and how many days remain until your next birthday.

The day-of-week calculation uses the Gregorian calendar's repeating structure, where each date drifts by one weekday per year (or two after a leap day). The age calculation subtracts the birth date from today, borrowing across months and years when the day difference is negative — the same fiddly arithmetic an age calculator performs.

The total-days figure is pure elapsed time: the number of midnights between your birth and today. The next-birthday count looks ahead to the same month and day in the current or following year, so it always shows a value between 0 and 365.