Date Calculator

How to use the date calculator

The date calculator does two jobs: it counts the time between two dates, and it adds or subtracts days, weeks, months or years from a date. It uses the real calendar, including leap years, and runs in your browser.

Days between dates

1. Choose Days between dates and pick a start date and an end date.
2. Decide whether to count both days. Tick Include the end date when both the first and the last day count, for example for a hotel stay or a holiday rota.
3. Read the result: the number of days, the same span in years, months and days, in weeks and in months, the number of weekdays (Monday to Friday) and weekend days, and the hours.

Add or subtract

1. Choose Add or subtract and pick the start date.
2. Choose + Add or − Subtract and enter years, months, weeks and days in any combination.
3. Tick working days if the days should skip Saturdays and Sundays, for example for a delivery or notice period.
4. Read the new date, its day of the week, its day number in the year and its ISO week number.

Counting the days between two dates

By default, the calculator counts the days from the start date up to the end date, the way most people count “how many days until”: from Monday to Friday is 4 days. With Include the end date ticked, both days count, so Monday to Friday is 5 days.

Worked example: from 1 March 2026 to 25 December 2026. March has 31 days, so there are 30 days left after 1 March, then April 30, May 31, June 30, July 31, August 31, September 30, October 31, November 30 and 25 in December: 30 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30 + 25 = 299 days, or 9 months and 24 days.

If the end date is before the start date, the calculator shows the difference as a positive number and tells you the dates were the other way round.

Weekdays, weekends and working days

For deadlines, notice periods and deliveries, the number of weekdays often matters more than the total. The “between” result shows weekdays (Monday to Friday) and weekend days separately. When adding days, tick working days to skip Saturdays and Sundays.

Public holidays are different in every country, region and company, so the calculator doesn’t remove them. If a holiday falls in your range, subtract it yourself; for example, 10 working days from a Monday with one public holiday in between ends one working day later.

Adding months: what happens at the end of a month

Months have different lengths, so “one month after 31 January” has no exact answer: there is no 31 February. The calculator keeps the same day number when it exists and otherwise moves to the last day of the month, and it tells you when it did. So 31 January + 1 month = 28 February (29 February in a leap year), and 31 March − 1 month = 28 or 29 February.

Years work the same way: 29 February 2028 + 1 year = 28 February 2029.

Day of the year and ISO week numbers

Planning calendars and many businesses use week numbers. The calculator gives the ISO week number of a date, the standard used across Europe and much of the world: weeks start on Monday, and week 1 is the week that contains the year’s first Thursday (see ISO week date). That’s why 1 January can fall in week 52 or 53 of the previous year. It also gives the day of the year, from 1 to 365 (366 in a leap year).

Checking a date difference by hand

For short ranges, counting on a calendar is fine. For longer ones, a quick method avoids mistakes: count the days left in the start month, add the full months in between using their real lengths (January 31, February 28 or 29, March 31, April 30, May 31, June 30, July 31, August 31, September 30, October 31, November 30, December 31), then add the day number of the end date. The worked example above uses exactly this method.

The two most common slips are forgetting that February has 29 days in a leap year, and counting both the first and the last day when you meant to count only one of them. The calculator avoids both, and its Include the end date option makes the choice explicit.

Why daylight saving doesn’t change the count

Some date calculators work in local time, so a day with a clock change counts as 23 or 25 hours and the total can be off by one. This calculator treats every date as a whole calendar day, so the number of days is always the number of calendar days, whatever time zone you’re in. The hours shown are simply days × 24.

Common date calculations

Ask in plain words

• “What date is 90 days from today?”
• “How many working days are there between 1 and 31 August?”
• “What week number is 15 October 2026?”

Type a question under the calculator or in the AI workspace. For percentages of a period, such as “what share of the year has passed?”, the percentage calculator helps.

Privacy

The dates you enter stay in your browser; nothing is uploaded, stored or used to train AI. More in Trust and privacy.

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