How to use the date difference calculator

Choose a mode. “Days between dates” compares two dates; “Add or subtract time” moves a date forwards or backwards.
In difference mode, pick the start and end dates. The start date defaults to today. If you enter them the wrong way round, the tool swaps them and tells you.
Decide about the end date. Leave the toggle off to count the gap between the dates; switch it on to count both endpoints, which is what “inclusive” means on most forms.
In date-maths mode, enter a starting date, choose add or subtract, then fill in any combination of years, months, weeks and days.
Read the results: total days, the same gap in years, months and days, weeks plus leftover days, business days and weekend days, and the weekday each date falls on.

How the calculation works

Dates are parsed as plain calendar dates — year, month and day — and compared in UTC, so leap days count correctly and nothing shifts with your time zone.

Total days is a straight subtraction, plus one if you include the end date:

total days = (end − start) ÷ 1 day  (+1 if the end date is included)

Years, months and days walk the calendar rather than dividing by an average month length. The tool adds whole months to the start date for as long as the result stays on or before the end date, then counts the leftover days. When a day of the month does not exist in the target month it is clamped to the last day.

Business days count every Monday to Friday in the range. Rather than looping over every date, the tool takes the whole weeks in the span, multiplies by five, and checks the handful of leftover days:

business days = whole weeks × 5 + weekdays among the remaining days

Toggle	2026-09-01 → 2026-09-30
End date excluded	29 days, 21 business days
End date included	30 days, 22 business days

Worked examples

A full calendar year. From 1 January 2024 to 31 December 2024 is 365 days — or 366 with the end date included, since 2024 has a leap day. In calendar terms that is 0 years, 11 months and 30 days, which is also 52 weeks and 1 day.

A month of work. September 2026 runs from Tuesday the 1st to Wednesday the 30th. Excluding the end date the range holds 21 business days; including it, 22. Across the whole of 2026 there are 261 weekdays, before any public holidays are taken off.

Payment terms. An invoice issued on 21 September 2026 with net-30 terms is due 30 days later, on 21 October 2026, a Wednesday. Stretch the same start date by 100 days and you land on 30 December 2026.

Month-end clamping. A subscription that starts on 31 January 2026 and renews monthly next bills on 28 February 2026, not 3 March. The following renewal returns to the 31st where the month allows it, which is why the tool applies months to the original date rather than chaining one clamped result into the next.

Tips and common mistakes

“Within 14 days” is usually inclusive of the deadline but not the start. Read the wording carefully before deciding which toggle to use; a single day matters for filing deadlines.
Nights and days are not the same. A trip from Friday to Sunday is two nights but three days. The include-end-date toggle is what switches between those two readings.
Business days are not working days everywhere. Weekends fall on Friday and Saturday in parts of the world, and this tool assumes Saturday and Sunday. Adjust manually if your week differs.
Adding 12 months is not always the same as adding a year plus a day. 29 February plus one year lands on 28 February, and adding a day afterwards gives 1 March — the order of operations changes the answer.
Historical dates. Countries adopted the Gregorian calendar at different times, so very old dates may not match contemporary records even though the arithmetic is correct.

Glossary

Inclusive count – a count that includes both the first and last day of a range.
Business day – Monday to Friday here, with no allowance for public holidays.
Clamping – moving a date back to the last valid day of a month when the original day does not exist there.
Calendar difference – a gap expressed as years, months and days rather than as a single day count.

Privacy

Everything is calculated in your browser with plain JavaScript. The dates you enter are never sent anywhere.

Frequently asked questions

How many days are between two dates?

Subtract the earlier date from the later one. From 1 January 2024 to 31 December 2024 is 365 days, or 366 if you count the end date as well, because 2024 is a leap year.

Should I include the end date?

It depends on what you are counting. A hotel stay or a rental period usually counts nights, so leave the toggle off. A notice period, a holiday allowance or a project that runs "through" a date usually counts both endpoints, so switch it on.

How are business days counted?

Every Monday to Friday inside the range is counted and weekends are skipped. Public holidays are not deducted, because they vary by country, region and industry — subtract your own from the total.

What happens when I add a month to 31 January?

The result is clamped to the last day of the shorter month, so you get 28 February, or 29 February in a leap year. Adding a month twice from 31 January gives 31 March, but adding two months at once gives 31 March too; the clamp only applies to the final month.

Does the order of years, months and days matter?

Yes, and this tool fixes the order: years and months are applied first, then weeks and days. That is the convention used by most date libraries and contracts, and it avoids ambiguous answers around month ends.

Do time zones affect the result?

No. Dates are treated as plain calendar dates in UTC, so a date never slips by a day because of your local clock or a daylight saving change.

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Last reviewed: September 21, 2026

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