Adding months is not the same as adding 30 days: one month after 31 January is 28 or 29 February (the last day of the month), and one month after 15 March is 15 April. This calculator adds months and years by calendar date, then the weeks and days.
How to use the Date Calculator – Add or Subtract Days
- Start date
- Operation
- Count
- Years
- Months
- Weeks
- Days
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose the start date, whether to add or subtract, and how many years, months, weeks and days – or switch to business days.
Tips
- Check whether the end date should be included – event durations usually include it, elapsed time does not.
- Months have 28–31 days, so “one month” is counted by calendar date, not as 30 days.
- Use 24-hour time or set AM/PM carefully when entering clock times.
- Time zones and daylight-saving changes are not applied unless the calculator says so.
Common mistakes to avoid
- Counting a 30-day month when a calendar month is meant (or the reverse).
- Forgetting leap years when counting days across February.
- Mixing decimal hours (7.5 h) with clock time (7:30).
Inputs explained
| Field | What to enter |
|---|---|
| Start date | A date |
| Operation | Choose one: Add (+), Subtract (−) |
| Count | Choose one: Calendar days, Business days (Mon–Fri) |
| Years | A number |
| Months | A number |
| Weeks | A number |
| Days | A number |
What each result means
- Day of the week = weekday([Resulting date])
- Calendar days from start = [Resulting date] – [Start date]
- ISO week number = (floor((dayofyear(([Resulting date]) – mod(weekday([Resulting date]) + 6, 7) + 3) – 1)÷7) + 1)
Formula
Result = start date ± years ± months (by calendar date) ± weeks × 7 ± daysBusiness days mode: step day by day, skipping Saturdays and SundaysWorked example
Inputs — Start date: 2026-10-25; Operation: Add (+); Count: Calendar days; Years: 0; Months: 0; Weeks: 0; Days: 90.
Results — Resulting date = Sat, January 23, 2027; Day of the week = Saturday; Calendar days from start = 90; ISO week number = 3.
Frequently asked questions
What date is 90 days from today?
Enter today's date and 90 days – the result appears instantly.
How do I add months to a date?
Move forward by calendar month, keeping the day number; if it doesn't exist (31 April), use the month's last day.
What is a business day?
Monday to Friday, excluding public holidays (which you can subtract separately).
What formula does the Date Calculator – Add or Subtract Days use?
Result = start date ± years ± months (by calendar date) ± weeks × 7 ± days.