How to add or subtract days, weeks and months to a date

Steps

1. Choose the start date. Here it is 1 January 2026.
2. Choose whether to add or subtract, the number and the unit: days, weeks, months or years.
3. Enter 100 days. The result is Saturday, 11 April 2026.
4. Check the weekday shown, which helps if the date falls on a weekend or a holiday.
5. To go backwards, choose subtract: 15 March 2026 minus 90 days is Monday 15 December 2025.

Weeks and months

Weeks are 7 days each, so 6 weeks after 1 January 2026 is 42 days later, Thursday 12 February 2026. Months and years keep the day of the month. 15 January plus 3 months is 15 April.

When the target month is shorter, the last day of that month is used. 31 January 2026 plus 1 month is 28 February 2026, and 29 February 2024 plus 1 year is 28 February 2025.

Where this helps

Use it for a notice period, a warranty or return window, a due date, a course end date or a treatment plan. Add the days of a 90-day period to the start date to see when it ends, or subtract days from a deadline to find when work must begin.

Check what the rule means

Contracts and laws sometimes count the start day as day one and sometimes as day zero, and some exclude weekends or holidays. The calculator counts days from the start date, so the start day itself is day zero. If the rule counts working days only, use the working days calculator and check the dates against the calendar.