Add or subtract days, months and years with a date calculator
The Date Calculator answers questions like “what date is 90 days from today?”, “what was the date 6 weeks ago?” and “when is 18 months after my start date?”. Pick a start date, choose add or subtract, enter years, months, weeks and days, and the result appears instantly with its weekday, ISO date, day of the year and week number.
Switch to Business days to count only Monday to Friday — useful for delivery estimates, notice periods, payment terms and legal deadlines. Quick buttons cover common jumps such as +30 days and +1 year. Everything is calculated in your browser.
How to use the Date Calculator
- Choose the Start date, or press Today.
- Select Add (+) or Subtract (−).
- In Calendar units, type any mix of years, months, weeks and days — or press a quick button such as +90 days.
- For working days, switch to Business days and enter how many Monday-to-Friday days to move.
- Read the resulting date and weekday. If the start day does not exist in the target month, a note explains the month-end rule that was applied.
- Press Use result as start date to chain another step, or Copy result to share it.
Date Calculator features
Years, months, weeks and days together
Enter several units at once, such as 1 year, 2 months and 10 days, in either direction.
Correct month ends
Adding a month to 31 January gives 28 or 29 February, never a date in March, and the tool tells you when that happens.
Business-day mode
Skips Saturdays and Sundays when moving forwards or backwards, like a WORKDAY formula.
Weekday, ISO date and week number
Every result shows the day of the week, the YYYY-MM-DD form, the day of the year and the ISO 8601 week.
Chain calculations
Turn the result into the new start date to build up schedules step by step.
How adding months and years works
Days and weeks are simple: a week is always 7 days, so adding them just moves along the calendar. Months and years are different because months have 28 to 31 days. The calculator adds the years and months together first — keeping the same day number — and only then adds weeks and days.
If the start day does not exist in the target month, the date is moved back to the last day of that month. This is the same month-end rule used by spreadsheet functions such as EDATE and by most programming libraries, and it means you never skip into the following month by accident.
| Start date | Change | Result | Why |
|---|---|---|---|
| 31 Jan 2024 | +1 month | 29 Feb 2024 | 2024 is a leap year; February has 29 days |
| 31 Jan 2023 | +1 month | 28 Feb 2023 | February 2023 has 28 days |
| 29 Feb 2024 | +1 year | 28 Feb 2025 | 2025 has no 29 February |
| 31 Mar 2024 | −1 month | 29 Feb 2024 | Clamped to the end of February |
| 31 Jan 2024 | +1 year 1 month | 28 Feb 2025 | Added as 13 months, then clamped |
| 1 Jan 2024 | +90 days | 31 Mar 2024 | 31 + 29 + 30 days later |
Business days versus calendar days
A business day here is any Monday to Friday. Adding 1 business day to a Friday gives the following Monday; adding 10 business days moves exactly two working weeks. Counting starts after the start date, so the start date itself is never counted. If you start on a Saturday or Sunday, the first business day counted is the next weekday in the direction you are moving.
Public holidays are not skipped because they differ between countries, states and companies — 26 January in India, 4 July in the US and bank holidays in the UK, for example. Add one business day for each holiday that falls in your range, or use the Business Days Calculator to count working days between two dates.
Common uses
- Deadlines and notice periods: 30 or 90 days from a letter, invoice or resignation date.
- Warranties, subscriptions and visas: the expiry date 1 year, 6 months or 180 days after purchase or entry.
- Project plans: chain several steps with “Use result as start date”.
- Looking back: subtract to find the date 100 days ago or the start of a 12-week period.
- To measure the gap between two known dates instead, use the Date Difference Calculator. To check a weekday on its own, try the Day of the Week Calculator.
Date Calculator FAQ
What date is 90 days from today?
Open the date calculator, leave the start date on today and press “+90 days”. The result shows the exact date and weekday. Ninety days is about three months, but not exactly, because months differ in length.
How do I add months to a date?
Type the number of months in the Months box. The day number stays the same, unless the target month is shorter — then the result is the last day of that month (for example 31 January + 1 month = 28 or 29 February).
Does the date calculator count the start date?
No. Adding 1 day to 10 March gives 11 March. Adding 0 days returns the start date itself.
How do business days work?
Business days count Monday to Friday only. Saturdays and Sundays are skipped, so Friday plus 1 business day is Monday. Public holidays are not removed automatically.
Can I subtract dates to find a past date?
Yes. Choose “Subtract (−)” and enter the years, months, weeks or days to go back. The month-end rule works the same way in reverse.
What is the ISO week number shown?
ISO 8601 weeks start on Monday, and week 1 is the week that contains the year’s first Thursday. That is why 30 December 2024 belongs to week 1 of 2025. See the Week Number Calculator for more.