Date Calculator: Difference, Add/Subtract Time, Find Day of Week

The JavaScript Date Calculator lets you (1) count days between dates, (2) add or subtract days, weeks, months or years, and (3) show the weekday for any date—all in one click. It applies the Unix-epoch millisecond formula and adjusts for leap years, so results stay accurate; the Gregorian calendar has 97 leap years every 400-year cycle (ISO 8601:2019).








