About the Add or Subtract Days from a Date
Find the date that is a number of days, weeks, months or years before or after another date, for example 90 days from today, 6 weeks before an exam or 3 months after a contract starts. You can also count only weekdays (Monday to Friday), which is how many delivery times and notice periods are given.
The result shows the full date with its day of the week, the ISO date (YYYY-MM-DD) to copy, and how many calendar days that is.
How it works
Days and weeks are simple: the calculator moves along the calendar by that many days (a week is 7). Leap years are included automatically, so adding 365 days does not always land on the same date next year.
Months and years are different, because months have different lengths. The calculator keeps the same day number in the new month. If that day doesn't exist (there is no 31 April), it uses the last day of the month and tells you. This is the same rule used by programming libraries such as Java's LocalDate.
For weekdays it steps one day at a time and counts only Monday to Friday. The start day itself isn't counted, so one weekday after a Friday is the following Monday.
date + n days = the date n days later date + n weeks = date + 7n days date + n months = same day number n months later, or that month's last day if shorter date + n years = date + 12n months
Assumptions and limits
- Dates use the Gregorian calendar, from year 1 to 9999.
- Weekdays skip Saturdays and Sundays only; public holidays are not skipped because they differ by country and employer.
- Adding 0 weekdays returns the start date unchanged, even if it falls on a weekend.
Worked examples
90 days from 13 October 2026
18 days take you to 31 October, 30 more to 30 November, 31 more to 31 December, leaving 11: the answer is Monday 11 January 2027.
One month after 31 January
There is no 31 February, so the result is the last day of February: 28 February in 2026, or 29 February in a leap year such as 2028.
5 weekdays after Saturday 17 October 2026
Counting Monday 19 to Friday 23 gives five weekdays, so the answer is Friday 23 October, six calendar days later.
Adding months to the end of a month (2026)
| Start date | Add | Result | Why |
|---|---|---|---|
| 31 January | 1 month | 28 February | February 2026 has 28 days |
| 31 March | 1 month | 30 April | April has 30 days |
| 30 April | 1 month | 30 May | 30 May exists |
| 31 May | −1 month | 30 April | April has 30 days |
| 29 February 2028 | 1 year | 28 February 2029 | 2029 is not a leap year |
How to use the Add or Subtract Days from a Date
- Choose the start date. Pick a date or press Use today.
- Choose add or subtract, the amount and the unit. Days, weeks, months, years or weekdays.
- Read the new date. Copy the ISO date or check the working to see how it was reached.
Frequently asked questions
What happens when I add a month to the 31st?
If the next month has no 31st, the result is that month's last day, for example 31 March plus one month is 30 April. A note appears under the answer whenever this happens, so you can adjust it if your contract or rule counts differently.
Is adding 12 months the same as adding 365 days?
Not always. Twelve months lands on the same date next year, while 365 days falls one day short of it whenever the period includes 29 February. If a rule says 'one year', use the years unit.
How are weekdays counted?
Starting from the day after your start date, each Monday to Friday counts as one; Saturdays and Sundays are skipped. So one weekday after Friday is Monday, and five weekdays after a Saturday is the following Friday.
Can I subtract to find a date in the past?
Yes: choose Subtract, or type a negative number. Ninety days before 13 October 2026 is 15 July 2026. Every unit works backwards the same way, including weekdays and months.
Does it handle leap years?
Yes. The calendar follows the Gregorian rule, with a 29 February in years divisible by 4, except century years not divisible by 400. Adding one year to 29 February 2028 gives 28 February 2029.
Sources
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