Add / Subtract Days
Add or subtract any number of days from a starting date and get the resulting date in plain and ISO format, with leap-year and month-end examples.
Results
Resulting date
- ISO
- 2026-10-09
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Educational estimates · not financial advice · free, no signup
Formula / how it works
Resulting date = start date ± N calendar days, accounting for month lengths and leap years. Negative N moves back in time.
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Detailed guide
What it is, how to use it, how to read the numbers, common mistakes, a worked example, formulas, and FAQs.
What this calculator is
Counting forward or backward by a number of days across months of different lengths is easy to get wrong in your head. This calculator takes a start date and a number of days, positive or negative, and returns the resulting date in a readable form and in ISO format. It counts calendar days, including weekends and holidays. If a deadline is in business days, count weekdays instead. The tool handles leap years, so adding days across 29 February works correctly. Unlike a calculator that always starts from today, you choose the starting date.
When to use / who it helps
- A contract deadline is a number of days after a signing date.
- You want to find a date 90 days before an event.
- You are working out a return or warranty window.
- You are planning a project milestone.
- You are working out a due date from a start date.
How to use
Steps
- Enter the start date.
- Enter the number of days; use a minus sign to go backward.
- Read the resulting date.
- Use the ISO output for spreadsheets and forms.
- Check whether the deadline counts the start day.
- For business days, use a weekday tool instead.
How to interpret results
- Calendar days include weekends and holidays.
- The start day is day 0, so adding 1 gives the next day.
- Negative numbers give earlier dates.
- Month lengths are handled automatically.
- Leap days are included where relevant.
Common mistakes & gotchas
- Counting the start date as day 1.
- Using calendar days for a business-day deadline.
- Forgetting that a negative number moves backward.
- Mixing date formats.
- Overlooking time-zone differences for deadlines.
Worked example
Worked example — 9 September 2026 plus and minus 30 days
- Plus 30: 21 days reach 30 September, and 9 more reach 9 October 2026.
- Minus 30: 9 days back reach 31 August, and 21 more reach 10 August 2026.
- Leap-year check: 28 February 2028 plus 2 days is 1 March 2028, because 2028 has a 29 February.
- In 2027, 28 February plus 2 days is 2 March.
Result: 9 September 2026 plus 30 days is 9 October 2026, and minus 30 days is 10 August 2026.
To load the form's own demo values, tap Try example above the Calculate button.
How to calculate / formula
Resulting date = start date ± N calendar days, accounting for month lengths and leap years. Negative N moves back in time.
Frequently asked questions
Does the tool count weekends?
Yes, it counts calendar days.
How do I subtract days?
Enter a negative number of days.
Is the start day included?
No. Adding 1 day gives the following day.
Does it handle leap years?
Yes.
How do I count business days?
Use a business-days tool.
How is this different from days from today?
You choose any starting date instead of starting from today.