CalcOak

Date Duration Calculator: count the time between two dates, or add a duration to a date

How to use this calculator

On the "Between two dates" tab, pick a start and an end date and the results update at once: the total number of days, the same span as weeks and days, as calendar months and days, and as years, months and days, plus how many of those days fall on a weekday. Whether the end date counts depends on the question. A hotel stay from Friday 3 July to Monday 6 July is three nights, so leave the box unticked, but if you are paid for every day on site, including the Monday, tick "include end date" to make it four. Contract deadlines usually work the exclusive way: "within 30 days of 1 June" means by 1 July.

The "Add or subtract" tab moves a date by a number of days, weeks, months or years and shows the day of the week for the result. Subtracting two weeks from Tuesday 10 March 2026 lands on Tuesday 24 February, which is handy for working out when to send a notice or post a parcel. If you enter the two dates the wrong way round, the tool swaps them and says so rather than showing a negative number.

Why use it rather than counting on a calendar

Days are easy to miscount by one, and months are worse because they are not all the same length. When the tool adds a month to 31 January it cannot land on 31 February, so it moves to 28 February (29 in a leap year) and tells you so. The same rule applies to 29 February plus one year. Spreadsheet EDATE and most billing systems behave the same way, which is why a subscription taken out on the 31st renews on the last day of shorter months. The "months and days" breakdown uses the same clamping, so 31 January to 1 March comes out as one month and one day rather than an odd negative figure.

The calculator deliberately ignores time zones and clock times. Every date is a whole calendar day, so a span that crosses the start of daylight saving time is never an hour short, and a date entered in Tokyo gives the same answer as in Los Angeles. Nothing is sent anywhere: the dates stay in your browser.

Further reading

Frequently asked questions

Does the number of days include both the start and the end date?+

By default the end date is not counted, so 1 March to 3 March is 2 days, the same answer you get by subtracting the dates in a spreadsheet. Tick "include end date" to add one and count every calendar day the span touches, which is what you want for "how many days am I away" or for counting days of a rental period.

How are months counted when the dates fall on different days of the month?+

The calculator steps forward whole calendar months from the start date first, clamping to the last day of a short month if it has to, and then counts the days left over. So 31 January to 1 March is one month and one day: one month lands on 28 February, and one more day reaches 1 March. That matches how people count rent periods, notice periods and subscriptions.

What happens when I add a month to the 31st?+

There is no 31 February, 31 April or 31 September, so the result is moved back to the last day of that month and the tool tells you it did so. Adding one month to 31 January 2026 gives 28 February 2026; in a leap year such as 2028 it would give 29 February. Adding a year to 29 February also clamps, to 28 February.

Why does the weekday count not match my business days?+

This page only separates Monday to Friday from Saturday and Sunday. It knows nothing about public holidays, and it does not let you change which days form the weekend. For a working-day count that removes national holidays, or to add a number of business days to a date, use the business days calculator linked below the tool.

Do time zones or daylight saving affect the result?+

No. The tool works entirely with calendar dates and treats every day as exactly one day, so the switch to or from daylight saving time never produces a 23-hour or 25-hour day in the count. The hours and minutes shown are simply the day count multiplied by 24 and by 1,440.

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