Date Calculator

Calendar arithmetic is not commutative, so the order the units are applied in changes the answer. This calculator applies years first, then months, then weeks and days, which is the order a contract, a notice period or a statutory deadline is read in. Adding one month and thirty days to 31 January is not the same as adding thirty days and then one month, and stating the order removes the ambiguity rather than hiding it.

The result names the weekday as well as the date, because the weekday is usually the point. A deadline that lands on a Saturday is a different problem from one that lands on a Wednesday, and you should not have to open a calendar to find that out.

Adding one month to 31 January has no literal answer, because 31 February is not a date. The calculator clamps to the last day of the target month, so the result is 28 February, or 29 February in a leap year. That is the convention used by civil registries, payroll systems and most legal drafting, and it means adding a month twice from 31 January gives 31 March rather than getting stuck at the end of February.

Leap years are applied in full: divisible by four, except centuries, except centuries divisible by four hundred. Subtracting works the same way in reverse, with the same clamping, so adding a span and then subtracting it returns you to a date in the same month even where the day of the month had to be adjusted on the way out. The calculator says when clamping has happened instead of leaving the difference for you to notice later.

With business days only turned on, the days and weeks you add are counted as Monday to Friday, skipping Saturdays and Sundays as it goes. That is how a working-days deadline is meant to be read — ten business days from a Thursday is two full weeks plus two days, not ten days on the calendar.

Public holidays are not skipped. They differ by country, by region and sometimes by employer, and a result that had quietly assumed one calendar would be wrong for most of the people reading it. Where holidays matter to your deadline, add them yourself, and check whether the rule you are working to counts them at all. Years and months are always calendar units even in business days mode, since no jurisdiction defines a month in working days. All of this runs in your browser from plain calendar dates, with no time zone applied and nothing uploaded. To measure the gap between two dates you already have rather than to land on a new one, use the days between dates calculator.