Date calculator: days between dates, add days, weekday and week number

Give the calculator two dates and it returns the gap in both calendar days and working days; give it one date and a number, and it moves forwards or backwards to land on the resulting date; give it a single date, and it names the weekday and the ISO week number. From 7 July 2026 to 25 December 2026 is 171 days, of which 123 are working days — the same span the three modes below draw on.

What each mode does

Days between two dates counts the gap from one date to another and splits it into calendar days and working days: 7 July 2026 to 25 December 2026 is 171 calendar days, or 123 working days.

Add or subtract days shifts a date by a number of days you type, forwards for addition or backwards for subtraction, and returns the date it lands on.

Weekday and week number takes a single date and tells you which day of the week it falls on and which ISO week of the year it belongs to.

A short span makes the calendar-versus-working distinction clear: from a Friday to the following Monday is 3 calendar days but only 1 working day.

Counting rules, stated plainly

The tool counts the gap exclusive of the start date and inclusive of the end date. So the count is the number of nights between the two dates, or equivalently the number of days you would tick off a calendar starting the morning after the first date.

Working days are Monday to Friday. Saturdays and Sundays are removed from the count, which is why a three-day Friday-to-Monday span collapses to a single working day.

Public holidays are not subtracted, because they differ by country and often by region. Count the holidays that apply to you and take them off the working-day figure yourself.

Week numbers

Week numbers here follow ISO 8601, the international standard for numbering weeks. Weeks run Monday to Sunday, and week 1 is the week that holds the year’s first Thursday. That rule means the start and end of a year can belong to a neighbouring year’s numbering.

A late-December date can fall into the next year’s week 1 if it sits in the week whose Thursday lands in January. The reverse happens too: an early-January date can belong to the previous year’s final week. So 30 December 2024 belongs to week 1 of 2025, while 1 January 2027 belongs to week 53 of 2026.

Most years have 52 weeks, but a few have 53 to keep the numbering aligned with the calendar. 2026 is one of them — a 53-week year in the ISO system.

Method note

Every result is pure calendar arithmetic on the Gregorian calendar, the civil calendar used almost everywhere today. Leap years are handled by the calendar’s own rules, so spans crossing 29 February count correctly. Nothing is sent anywhere: the calculation runs in your browser.