How to use the add or subtract days calculator
- Choose the start date. It opens on today.
- Pick Add to look forward or Subtract to look back, type the amount, and choose days, weeks, months or years.
- Tick Count weekdays only if Saturdays and Sundays should not count, for example for a delivery window quoted in business days.
The answer shows the full date with its weekday, the ISO format (year-month-day), the number of calendar days between the two dates, and the day of the year. If you only need the gap between two known dates, use the date difference calculator.
How the date arithmetic works
Days and weeks are plain counting: a week is exactly seven days, and the calendar rolls over month ends and leap days on its own. Months and years are different, because months have different lengths:
new month = start month + n (carrying into the next year) new day = start day, but no later than the last day of the new month
That second line is the important rule. Adding one month to 31 January cannot give "31 February", so the result is the last day of February: 28 February 2025, or 29 February in a leap year. Adding one year to 29 February 2024 likewise lands on 28 February 2025. The tool tells you whenever this adjustment happens.
Weekday mode moves one day at a time and only counts Monday to Friday. Starting on a Friday and adding one weekday gives the following Monday.
Worked examples
- 45 days after 15 January 2025. January has 16 days left after the 15th (reaching 31 January), which leaves 29; February 2025 has 28 days (reaching 28 February with 1 left), so the answer is Saturday, 1 March 2025.
- 10 weekdays after Friday, 3 January 2025. Ten Monday-to-Friday days is two working weeks, so the date is Friday, 17 January 2025, 14 calendar days later.
- 1 month after 31 January 2025. February has no 31st, so the result is 28 February 2025.
- 90 days before 1 July 2025. That is 2 April 2025: 30 days back reaches 1 June, 61 reaches 1 May (31 days in May), and 90 reaches 2 April.
Everyday uses
| Question | Setting |
|---|---|
| When is the 30-day return window over? | Add 30 days to the purchase date |
| What is the due date of a 14-day invoice? | Add 14 days to the invoice date |
| When does a 6-month contract renew? | Add 6 months to the start date |
| When will a parcel arrive in 5 working days? | Add 5 days, weekdays only |
| What date was 100 days ago? | Subtract 100 days from today |
| How far is 40 weeks from a given date? | Add 40 weeks |
For legal deadlines, check how the relevant rule counts. Some courts and contracts exclude the first day, skip public holidays, or move a weekend deadline to the next business day. This tool counts plain calendar days unless you choose weekdays.
Month lengths and why months are tricky
| Month | Days | Month | Days |
|---|---|---|---|
| January | 31 | July | 31 |
| February | 28 (29 in a leap year) | August | 31 |
| March | 31 | September | 30 |
| April | 30 | October | 31 |
| May | 31 | November | 30 |
| June | 30 | December | 31 |
Because of the uneven lengths, "30 days" and "1 month" are different things. Thirty days after 1 January is 31 January, but one month after 1 January is 1 February. Contracts and subscriptions often say "monthly" while loan notices say "30 days", so pick the unit that matches the wording you are checking. When you add months, the day of the month is kept whenever possible, so 15 March plus 3 months is 15 June, whatever the lengths in between.
Subtracting months works the same way in reverse. Subtracting 3 months from 31 January 2025 gives 31 October 2024, but subtracting one month from 31 March gives 28 February (or 29 in a leap year), the last day of the shorter month. Because of this clamping, adding a month and then subtracting a month does not always return you to the starting date. Starting from 31 January, adding one month gives 28 February, and subtracting one month from that gives 28 January, not 31 January.
Accuracy and limits
Dates follow the Gregorian calendar and are handled as whole days with no time zone, so daylight-saving changes never shift the answer. Weekday mode does not know about public holidays, which differ by country and year. The amount is limited to 100,000 and results must fall between the years 1 and 9999.
Frequently asked questions
What is 90 days from today?
Choose today as the start date, leave the operation on Add, enter 90 and keep the unit on days. The result shows the exact date and its weekday.
Why did adding one month to 31 January give 28 February?
February has no 31st, so the calculator uses the last day of the month instead. In a leap year the result is 29 February.
Can I skip weekends?
Yes. Tick the weekdays-only box. It counts Monday to Friday only, but it does not know about public holidays.
Is a week always seven days?
Yes. Adding weeks is the same as adding seven times that number of days, with no adjustment.
Does this handle daylight saving time?
Dates are calendar days, not clock times, so a clock change never moves the result. It only matters if you also need an exact hour, in which case use the time zone converter.