Date Calculator
Add or subtract days, weeks and months from any date. Free, instant.
What Is a Date Calculator?
A date calculator lets you add or subtract days, weeks, months or years from any date, and find the difference between two dates. Calendar arithmetic sounds simple but is surprisingly tricky — months have different lengths, leap years add an extra day, and business day calculations need to skip weekends. This tool handles all of that automatically, giving you accurate results for deadline calculations, contract periods, project planning, SLA tracking and more.
Adding and Subtracting Dates
Use a positive number to move forward in time and a negative number to move backward. Adding months accounts for variable month lengths — adding one month to January 31 gives February 28 (or 29 in a leap year), not March 2 or 3. Adding years similarly handles the February 29 edge case for leap year dates.
Business Days vs Calendar Days
Calendar days count every day including weekends. Business days (Monday–Friday) skip Saturday and Sunday. Use the Skip weekends option when calculating deadlines, SLAs, or working-day periods. Note that public holidays are not automatically excluded — add those manually if needed for strict business-day counts.
Common Date Arithmetic Examples
| Task | How to use this tool |
|---|---|
| When is 90 days from today? | Set start = today, unit = days, amount = 90 |
| How long until my deadline? | Date difference: start = today, end = deadline |
| 6-month contract end date | Set start = contract start, unit = months, amount = 6 |
| 30 business days from signing | Enable skip weekends, amount = 30 days |
| How many weeks between two dates? | Date difference tab — shows weeks alongside days |
Why Date Arithmetic Is Harder Than It Looks
Adding 30 days to a date is straightforward, but "add 1 month" is ambiguous: does January 31 + 1 month become February 28 or March 2? Different applications answer this differently. This tool uses the end-of-month clamp rule — the result is the last valid day of the target month — which matches the behaviour of most spreadsheet applications and ISO 8601 arithmetic. Similarly, leap years affect any date in February: adding 1 year to February 29 lands on February 28 in non-leap years. If you need strict day-count arithmetic, use the Days unit rather than months or years.