Enter your full birth date (year, month, day) and the calculator returns the weekday using the proleptic Gregorian calendar. Internally it relies on the same ISO 8601 day-numbering rule that browsers use, so the result matches reference tables for any date from 1582 onward.

Age is the number of completed years between your birth date and today, then the leftover months and days. The calculator subtracts a year if today is before your birthday in the current year — the same convention used on passports and legal forms.

A year is a leap year if it is divisible by 4, except century years, which must also be divisible by 400. So 2000 was a leap year, 1900 was not, and 2024 was. Leap years add February 29, giving 366 days instead of 365.

In non-leap years the calculator treats March 1 as your nominal birthday for age-rollover purposes, matching the majority legal convention in the United States and the United Kingdom. Your true anniversary still falls only every four years.

After you calculate, the tool tells you the weekday of your upcoming birthday. Because 365 is one day more than 52 weeks, your birthday shifts forward by one weekday each year, or by two days when a leap day intervenes.

The calculator uses the proleptic Gregorian calendar, extending today's rules backward in time. Dates before October 15, 1582 will return mathematically valid weekdays, but they will not match the Julian calendar that was in actual use at the time.

No. All math runs in your browser using JavaScript Date objects. Nothing is sent to a server, logged, or saved between visits — close the tab and the input is gone.