Date Calculator

Perform date arithmetic — add or subtract days, weeks, months, or years from any date, or find the exact difference between two dates in multiple units.

Useful in everyday scenarios: contract lawyers computing notice periods (e.g., "60 days from today"), HR reps tracking probation end dates, event planners counting down to a launch, and developers verifying that a date token in a test fixture lands on the weekday they expected. Each of these breaks down easily in human terms but is tedious to work out manually because month lengths vary and leap years shift the count by a day once every four years (except century years not divisible by 400 — a rule Pope Gregory XIII introduced in 1582 and which still catches out back-of-envelope math).

Under the hood, arithmetic is performed using the native JavaScript Date object in UTC to avoid timezone-drift bugs. Adding months uses the "clamped last-day-of-month" convention: adding one month to January 31 gives February 28 (or February 29 in a leap year) rather than March 3, matching the behavior of Excel's EDATE and Python's dateutil relativedelta. Adding years also clamps: February 29 plus one year becomes February 28 in non-leap years. The difference mode returns results in years, months, weeks, days, hours, minutes, and seconds simultaneously so you can pick the unit that makes sense for your context.

Compared to doing this in Excel with DATEDIF or a spreadsheet formula, the calculator handles the edge cases automatically and works on any device without the Excel compatibility quirks that still plague DATEDIF on newer versions. Pair with Days Between Dates for quick day counts or Day of the Week Calculator to find what day a date falls on.