Date Calculator

Date Calculator answers the date questions that come up at work and at home. How many days until a deadline? How many working days does a notice period cover? What date is 90 days or ten working days from today, and which week number is it in?

Everything sits in three tabs, so you never need a second tool. “Days between” gives the total in days, a years, months and days breakdown, weeks, and the number of working days. “Add / subtract” moves a date by any mix of years, months, weeks and days, or by working days only. “Week & day” shows the weekday, ISO and US week numbers, the day of the year and a month calendar you can click through.

Month maths is handled honestly. Add one month to 31 January and you land on 28 February, or 29 in a leap year, and the tool says so instead of quietly skipping into March. You choose your own working week (Sunday to Thursday works too) and paste in your own public holidays, because holidays differ by country and state. All of it runs in your browser and nothing is sent anywhere.

1. Choose a tab: “Days between”, “Add / subtract” or “Week & day”.
2. Pick your dates with the calendar boxes, or press “Today”. The quick buttons such as “+30 days” or “End of the year” fill the end date for you.
3. For working days, open “Working week and holidays”, tick the days you work and type or “Add holiday” for each day off.
4. In “Add / subtract”, choose Add or Subtract and Calendar days or Working days, then type the years, months, weeks and days.
5. Read the answer as you type, then press “Copy result” to paste it into an email or calendar note.

• Days between two dates, with an option to count both the start and end day.
• A years, months and days breakdown, plus weeks, total months and hours.
• Working days that skip your chosen weekend days and your own holiday list.
• Holidays typed as 2026-12-25, 25 Dec 2026 or December 25, 2026, with a label after the date if you like.
• Add or subtract years, months, weeks and days in one go, or a number of working days.
• Clear month-end rules: 31 January + 1 month = 28 or 29 February, explained on screen.
• ISO 8601 week number and the US-style week number, day of the year, days left, quarter and leap year.
• A clickable month calendar with week numbers, and a finder that turns “week 41 of 2026” into its Monday to Sunday dates.

• “Days between” counts the start day but not the end day, like most planners. For “how many days am I on leave”, tick “Include the end day”.
• A holiday that falls on one of your weekend days is not subtracted twice. If your country moves it to the Monday, add the Monday as well.
• Working days in “Add / subtract” never count the start day itself, so 1 working day after a Friday is the following Monday.
• Week numbers differ around New Year. In the ISO system used across Europe and Australia, 30 December 2024 is week 1 of 2025; the US count starts a new week 1 on 1 January.
• When you add months and days together, months go first. 31 January + 1 month + 1 day gives 1 March in a normal year, not 29 February plus one.