Add or Subtract Days from a Date

Details

About this tool

Adds or subtracts a period from a date and names the weekday it lands on. Days, working days, weeks, months and years are all available, and working days are counted properly rather than approximated.

The deadline case is the one that catches people. Fourteen days from today and fourteen working days from today are different dates, usually by four days or more, and contracts, notice periods and court deadlines are routinely written in one while being calculated as the other. Choosing the unit explicitly removes that guess, and public holidays are skipped as well as weekends when the working-day unit is selected.

Adding months is the other place where a reasonable-looking answer can be wrong. 31 January plus one month has no obvious value, because 31 February does not exist. The convention used here - and by most software, including spreadsheets and standard date libraries - is to clamp to the last valid day, giving 28 or 29 February. It is worth knowing that this makes the operation asymmetric: add a month to 31 January and subtract one from the result and you do not return to 31 January.

Years behave the same way, so a year after 29 February is 28 February in a common year. The weekday of the result is shown because it is very often the actual question - a deadline landing on a Saturday usually is not the deadline.

All of it is calendar arithmetic on day numbers with no time zone and no time of day, so the result cannot slide by a day depending on where you are, and it runs in your browser.

Good to know

Questions people actually ask

What happens to 31 January plus one month?

It clamps to the last day of February - the 28th, or the 29th in a leap year - because 31 February does not exist. This is what spreadsheets and standard date libraries do. Note that it makes the operation asymmetric: subtracting a month from the result does not bring you back to 31 January.

What is the difference between days and working days?

Working days count Monday to Friday and skip public holidays; plain days count everything. Fourteen days and fourteen working days are usually four or more days apart, which is why the unit is an explicit choice - a notice period written in one and calculated as the other is a real and expensive mistake.

Which public holidays are used?

Polish public holidays, including the moveable ones - Easter Sunday and Monday, Corpus Christi - computed rather than listed from a table, so the calculation stays correct in future years. Turn the holiday option off to count weekdays only.

Can I subtract as well as add?

Yes - enter a negative amount. The same rules apply in reverse, including the month-end clamping, so subtracting a month from 31 March gives 28 or 29 February.

Why does the weekday matter?

Because it is usually the real question. A deadline that lands on a Saturday is generally not the deadline, an appointment on a public holiday is not an appointment, and a delivery date on a Sunday is not a delivery date. Showing it saves a trip to a calendar.