Date Calculator Guide
Add or subtract days, weeks, months and years, or measure the duration and business days between two dates.
Dates can be counted in two ways: measure the gap between two dates, or move a date forward or backward. Date Calculator has one mode for each question.
Count days between two dates
Leave Mode on Difference between two dates. Set Start date to 2024-01-01 and End date to 2024-01-03. Total days should say 2: from the start of January 1 to the start of January 3 is two days. Turn on Include the end date and it should say 3, counting January 1, 2, and 3. The Calendar difference describes the gap and stays at two days.
If you choose Count business days (Mon–Fri), the tool counts weekdays, not public holidays. A business day in your country may still be a holiday.
Move a date forward
Change Mode to Add or subtract from a date. Set Start date to 2024-01-31, Operation to Add, Amount to 1, and Unit to Months. The Result should be 2024-02-29. February does not have a 31st, so the tool uses its last valid day. In 2024 that is February 29 because it was a leap year.
The tool starts with dates near today; replace them to repeat these examples. Check which mode is selected before interpreting a result. These are calendar calculations on the device running your browser, not scheduled changes to an account or server.
Where your input goes
Processing runs in your browser. The tool does not upload your input to ilham.dev.
Related guide
For more background, read Dates, Timestamps, and Time Zones.
Questions you might have
- Why is the calendar difference not just the total days?
- A month is not a fixed number of days, so 31 Jan to 1 Mar is one month and one day, not 29 days. The calendar figure keeps the day of the month and clamps to the last valid day when a month is shorter, which is what people mean by a one-month difference.
- Does including the end date change the calendar difference?
- No. Include the end date adds one day to the total-day counts, which is what you want for a stay that starts and ends on the listed days, but the years/months/days figure still describes the gap between the two dates themselves.
- How are business days counted?
- Weekends (Saturday and Sunday) are skipped. No public-holiday calendar is applied, because holidays differ by country and year, so subtract them yourself if it matters.
- What happens when I add a month to 31 January?
- The day clamps to the last day of the target month, so 31 Jan + 1 month is 28 February, or 29 February in a leap year. Adding one month twice is not the same as adding two months, which is why the page does it in one step.