Add or Subtract Dates

Add or subtract years, months, weeks and days from any date and get the result with its weekday. Month ends are handled sensibly: add one month to 31 January and you land on the last day of February, rather than on a date that doesn't exist.

How to use it

1. Choose a start date.
2. Pick add or subtract.
3. Enter the years, months, weeks and days.
4. Read the new date.

When it helps

• Working out a deadline or due date.
• Finding a contract end date.
• Planning events a set time ahead.
• Seeing what the date was some time ago.

Good to know

• Years and months are applied first, then weeks and days.
• If the target month is shorter, the day moves back to its last day.
• Leap years are handled correctly.

How the calculation works

Years and months are added first, counting months across the calendar. If the day doesn't exist in the target month, such as 31 in a 30-day month, it moves back to the last valid day. Weeks and days are then added as exact multiples of 24 hours.

Because dates are calculated in UTC, daylight saving changes can't shift the answer by a day.

Worked example: contract dates

A 90-day notice period that starts on 12 January ends on 12 April: enter 12 January, add 90 days, and read 12 April (in a non-leap year). Add 3 months instead and you get 12 April as well, but the two can differ in other months, because months have different lengths.

A 6-month probation starting on 31 August ends on 28 February, or 29 in a leap year, not 31, because February has no 31st.

Common date traps

The word "month" is ambiguous in contracts, so check whether it means a calendar month or thirty days. Another trap is counting inclusively: from Monday to Friday is four days apart, but five days if you include both ends.

Business days, public holidays and time zones add further complexity. For working-day calculations, use the Business Days Calculator.

More examples to try

Subtract 18 years from today's date to find the latest birth date for an adult. Add 14 days to a purchase date to see a return deadline. Add 1 year and subtract 1 day to find the end of a yearly licence that starts today and runs for twelve months.

To plan a project, add 6 weeks to the kick-off date, and then use the Business Days Calculator to see how many working days that is once holidays are counted.

Checking the answer

After you get a date, use the Day of the Week Finder to see which weekday it lands on. If it falls on a weekend and your deadline needs a working day, the Business Days Calculator will move it for you.

Questions