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Date & Time

Add / Subtract Days

Add or subtract years, months, weeks and days to or from a date and see the resulting date and weekday, with an option to count business days only.

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Operation
Amount
Leave a box empty to use zero. Years and months are applied first, then weeks and days.
Applies to the Days box only. Public holidays are not skipped.

How to use the calculator

Choose a start date (it is set to today when the page opens), pick Add or Subtract, and fill in any of the Years, Months, Weeks and Days boxes. Empty boxes count as zero, but at least one amount must be more than zero. Press Calculate to see the resulting date together with its day of the week.

This is useful for deadlines such as "30 days from the invoice date", notice periods, warranty end dates, or finding out what date it was 90 days ago.

How the date is worked out

The amounts are applied in a fixed order:

  • First the years and months are added or subtracted together.
  • Then the weeks and days are added or subtracted.

The order matters because months have different lengths. When moving by months lands on a day that does not exist, the date is set to the last day of that month. January 31 plus 1 month is February 28, or February 29 in a leap year. In the same way, February 29, 2024 plus 1 year is February 28, 2025.

Worked example

Start on January 31, 2026 and add 1 month and 1 day.

  • Adding 1 month gives February 28, 2026, because February 2026 has only 28 days.
  • Adding 1 day then gives Sunday, March 1, 2026.

Adding 1 day first and then 1 month would have given a different answer (March 1 plus 1 month is April 1), which is why the calculator always uses the same order.

Business days

Tick Count the days as business days to skip Saturdays and Sundays. This option applies to the Days box only; years, months and weeks are still calendar periods.

  • Friday plus 1 business day is the following Monday.
  • If the start date is a weekend day, counting begins from the nearest working day, so Saturday plus 1 business day is Monday and Sunday minus 1 business day is Friday.
  • Public holidays are not skipped, because they depend on where you live.

Good to know

  • Each week is exactly 7 days, and leap days are counted automatically when days are added.
  • Results must fall between year 1 and year 9999. Larger amounts show an error instead of an incorrect date.
  • Time zones and daylight saving time do not affect the result, since only calendar dates are used.

Frequently asked questions

What is January 31 plus one month?

February 28, or February 29 in a leap year. When the same day number does not exist in the target month, the calculator moves to the last day of that month instead of spilling over into March.

In what order are years, months, weeks and days applied?

Years and months are applied first, then weeks and days. Because months have different lengths, a different order can give a different date, so the calculator always uses this one.

Does the business days option skip public holidays?

No. It only skips Saturdays and Sundays, and it applies only to the Days box. If a holiday falls inside your period, add one extra business day for each holiday yourself.

How do I find the date a number of days ago?

Choose Subtract, enter the number of days in the Days box and press Calculate. The start date defaults to today, so the result is the date that many days in the past.