Date Calculator
A date calculator with three modes — the difference between two dates, adding or subtracting a duration, and counting working days — each showing the ISO week number, day of year and weekday for every date involved.
How to use Date Calculator
- Choose a mode: difference between dates, add or subtract a duration, or working days.
- Enter your dates. Every date shows its weekday, ISO week number and day of the year.
- For business days, pick which days count as your weekend and paste in any public holidays to exclude.
- Copy the result, or download the working-day breakdown as CSV.
About Date Calculator
Date arithmetic is where mental maths reliably fails. Months have different lengths, leap years interrupt the pattern, and “three months from now” means something different depending on where in the month you start. This calculator handles the three questions that account for almost all date maths: how far apart are two dates, what date is a given duration from here, and how many working days lie between two points.
The difference mode gives both representations, because both are useful and they are not interchangeable. The total number of days is what you need for interest, notice periods and deadlines; years, months and days is what you need to describe a duration to a human. Alongside them sit total weeks, hours and minutes, and a business-day count that excludes your chosen weekend plus any public holidays you paste in — one date per line, in any common format.
Every date on the page carries its weekday, its ISO 8601 week number and its day of the year, because those are the details that turn a date into a plan. Adding durations clamps to the end of the month rather than overflowing, matching spreadsheet and legal convention: one month after 31 January is 28 February, not 3 March. Leap years follow the full Gregorian rule, so the year 2000 counts as a leap year and 1900 does not — the case that famously broke a generation of spreadsheets.
Frequently asked questions
How many days are there between two dates?
The tool counts whole calendar days between the dates, and also expresses the gap in years, months and days. Whether the end date itself counts is a choice — the “include end date” option is there because contracts and notice periods differ on this, and getting it wrong shifts every deadline by one day.
What counts as a business day?
By default Monday to Friday, excluding Saturday and Sunday and any dates you paste into the holiday list. You can change which days count as your weekend, which matters in countries where the weekend is Friday–Saturday or Thursday–Friday.
What is an ISO week number?
The ISO 8601 week of the year, where weeks start on Monday and week 1 is the week containing the first Thursday of January. It is the numbering used across European business, project planning and most enterprise software, and it occasionally puts late December into week 1 of the following year.
How does adding months to a date handle month ends?
By clamping. Adding one month to 31 January gives 28 February (or 29 in a leap year) rather than rolling into March, which is the convention used by spreadsheets, most legal drafting and every major date library.
Does it handle leap years correctly?
Yes. Leap years follow the full Gregorian rule — divisible by 4, except centuries, unless divisible by 400 — so 2000 was a leap year and 1900 was not.
Is it free and private?
Yes, free with no sign-up, and every calculation runs locally in your browser.
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