Date Calculator
Find the number of days between two dates, or add/subtract days, weeks, months or years from a date.
How to Use
- Choose which calculation you need from the options above the form.
- For "Difference between dates", pick both dates using the date pickers.
- For the add/subtract calculations, pick a start date and enter a whole number — use a negative number to subtract instead of add.
- The result updates instantly as you fill in the form.
Formula
- Difference between dates
- span = later_date − earlier_date, broken into whole years, whole months, and remaining days
- Add/subtract days
- result_date = start_date + days
- Add/subtract weeks
- result_date = start_date + (weeks × 7) days
- Add/subtract months
- result_date = start_date with `months` added to the calendar month, clamped to the last real day of the resulting month
- Add/subtract years
- result_date = start_date with (years × 12) added as months, using the same clamping rule
Worked Example
One month after January 31, 2023.
- The target month is February 2023.
- 2023 is not a leap year, so February only has 28 days — day 31 doesn't exist there.
- The day is clamped to the last real day of February: the 28th.
Result: Tuesday, February 28, 2023
About This Tool
What this tool does
This calculator finds the exact span between two dates, or a new date reached by adding or subtracting days, weeks, months or years from a starting date.
When to use it
Use it to count down to (or back from) an event, work out a deadline, or check exactly how long has passed between two dates.
What the result means
The "Difference between dates" result shows an unsigned span — years/months/days elapsed, plus the same span as a total day and week count — regardless of which date you entered first. The add/subtract results show the resulting calendar date, including the day of the week.
Assumptions & limitations
Add/subtract amounts must be whole numbers. When adding months or years pushes the day-of-month past the end of the target month (e.g. January 31 plus one month), the result is clamped to the last real day of that month rather than rolling over into the following month — so January 31 + 1 month lands on February 28 (or 29 in a leap year), not March 1 or 3. Adding twelve months and adding one year always produce the same result, since "add a year" is defined as "add twelve months."