Day of the Week Calculator

1. Day of Week tool: Pick any date using the date input. The calculator instantly shows you which day of the week that date falls on, along with additional facts: the day number within the year, how many days are left in that year, the ISO week number, and the calendar quarter.
2. Birthday calculator: Enter your birthdate to find out what day of the week you were born, your age in days, and your age in years. Also works for any historical or future date you want to look up.

JavaScript's built-in Date object uses the Unix timestamp (milliseconds since January 1, 1970 UTC) and the Gregorian calendar to determine the day of the week. The getDay() method returns 0 (Sunday) through 6 (Saturday).

ISO week number (used in Europe and for business planning) is calculated as:

1. Adjust date to nearest Thursday
2. Find which week that Thursday falls in for the year
3. Return week number 1-53

ISO week 1 is the week containing the first Thursday of the year. This means January 1 can sometimes be in week 52 or 53 of the previous year.

Day of year: Count the days from January 1 to the given date. January 1 = day 1, December 31 = day 365 (or 366 in a leap year).

Leap year rule: A year is a leap year if it is divisible by 4, except century years (divisible by 100) must also be divisible by 400. So 2000 was a leap year, 1900 was not, and 2100 will not be.