How to use the date calculator
- Choose whether you are measuring between two dates or adding a period to one.
- Pick the start date, and the end date if you are measuring an interval.
- For adding, choose add or subtract and fill in the days, weeks, months or years.
- Read the totals, including the working-day count with weekends removed.
- Copy the breakdown if you need it for a contract or a project plan.
What this calculator handles
- Days, weeks and months between any two dates, in either order.
- Working days with Saturdays and Sundays excluded.
- Calendar difference expressed as years, months and days.
- Adding or subtracting any combination of days, weeks, months and years.
- The weekday each date falls on, which deadlines often depend on.
- Dates handled locally, so nothing about your schedule is transmitted.
Why date arithmetic is harder than it looks
Counting days sounds trivial until leap years, unequal month lengths and daylight saving get involved. February has 28 days except when it has 29. Adding one month to 31 January has no obvious answer. Adding 24 hours across a daylight saving boundary does not always land on the same clock time the next day.
This calculator sidesteps the last problem by working in whole calendar days rather than elapsed hours, which is what people mean when they ask how many days apart two dates are.
Inclusive or exclusive counting
This trips people up constantly, and it matters legally. The interval from Monday to Friday is four days if you count the gaps, and five days if you count the days themselves.
This calculator counts the gaps, the difference between the dates. A contract saying "within 5 days" usually means five clear days after the start, so add one to the count if the starting day itself is included. When a deadline matters, check how the contract defines it rather than assuming, because both conventions are in common use.
Working days and what they miss
The working-day figure excludes Saturdays and Sundays and nothing else. Public holidays are not deducted, because they differ by country, by region within a country, and by year.
For UK business days you would typically subtract eight bank holidays a year, though Scotland and Northern Ireland differ from England and Wales. For contracts specifying "business days", the definition is usually in the agreement and often excludes named holidays. Use this figure as a starting point and adjust for the calendar that applies.
Adding months is genuinely ambiguous
- 31 January plus one month: 28 February, 2 March, or an error, depending on the system.
- 29 February plus one year: 28 February or 1 March, depending on the convention.
- Adding a month twice is not always the same as adding two months at once.
This calculator clamps to the end of the target month, so 31 January plus one month gives 28 February. That is the most widely used convention and it matches what most spreadsheet software does, but if a legal deadline hangs on it, confirm which rule applies.
Where this comes up
Notice periods, contract terms, project timelines, visa durations, warranty windows, notice to quit, and working out how long until a deadline. All of them reward being precise rather than approximate, and most of them are defined in working days rather than calendar days.
For an age rather than an interval the age calculator handles the year-month-day breakdown, and the time calculator works with hours and minutes within a day. Everything is calculated in your browser.
One last habit worth adopting: write dates in ISO order, as 2026-03-14. It sorts correctly as text, it is unambiguous between British and American readers, and it removes the class of error where 03/04 means two different days depending on who is reading it.