Day of the Week Calculator
Find the day of the week for any date. Also shows the day's position in the year, ISO week number, and days until the next occurrence of the same weekday.
About the Day of the Week Calculator
This calculator determines the day of the week for any date using JavaScript's native Date API. It also computes the ISO 8601 week number — an international standard where Week 1 is defined as the week containing the first Thursday of the year (or equivalently, the week containing January 4th). ISO weeks always start on Monday. If you need to count the number of days between two specific dates, try the Day Counter Calculator, which supports both inclusive and exclusive counting.
The "day of year" (also called the ordinal date) tells you the sequential position of the date within the year — January 1 is day 1, and December 31 is day 365 (or 366 in a leap year). This is commonly used in astronomy, accounting, and Julian date conversions. If you're scheduling events and need to convert times across different locations, the Time Zone Calculator shows equivalent times in every major world time zone.
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For a related calculation, use Hours Calculator to calculate total hours worked between two times, with optional break deduction. Handles midnight crossover. Alternatively, use Day Counter to count days between two dates or days until/since a specific date. Inclusive or exclusive counting.
For the underlying method, read Date Math Explained: Counting Days, Adding Time, and Finding Weekdays.