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Date Calculator: Add or Subtract Time from a Date
Start from any date, then add or subtract years, months, weeks and days. The result shows the new date and its weekday, and months that are too short are clamped to their last day.
How dates are added and subtracted
The calculator works in two steps. First it moves the date by whole years and months, treating a year as 12 months. Then it moves the result by weeks and days, counting a week as exactly 7 days. When subtracting, both steps run in the opposite direction, and the months step still happens first.
Days are exact calendar days, so leap years and the differing month lengths are handled for you. Internally every date is a UTC midnight, which means daylight saving changes cannot add or remove an hour and push the answer onto the wrong day.
What happens at the end of a month
Months have 28 to 31 days, so "one month later" is not always defined. If the target month is shorter than the start day, the result is clamped to the last day of that month. Adding one month to 31 January gives 28 February in 2026 and 29 February in 2024. Adding one month to 30 April gives 30 May, because May is long enough.
Clamping has a side effect worth knowing: it is not reversible. Adding a month to 31 January and then subtracting a month from the answer returns 28 January, not 31 January. If you need a strict day count instead, enter days or weeks and leave months at zero.
Typical uses
- Deadlines: find the date 90 days after a contract is signed or an invoice is issued.
- Planning: work out the date six months before an event for booking milestones.
- Pregnancy and health windows: add 280 days to a start date for a simple calendar estimate, bearing in mind that real timing varies.
- Anniversaries: add years to a start date to see which weekday an anniversary lands on.
If you want the gap between two dates instead of a new date, use the days between dates calculator. To skip weekends and holidays, use the business days calculator.
Limits and assumptions
The calculator uses the Gregorian calendar for every date, including years before it was adopted in various countries, so very old dates may not match historical records. Results are limited to the years 1000 to 9999. Legal and contractual deadlines sometimes count differently, for example excluding the start day or rolling a weekend deadline to Monday, so check the wording that applies to you.
Worked example
What is one month after 31 January 2026? February has only 28 days that year, so the answer is clamped to the last day of February.
Inputs
| Start date | 2026-01-31 |
|---|---|
| Operation | Add to the date |
| Years | 0 |
| Months | 1 |
| Weeks | 0 |
| Days | 0 |
Result
Added to 31 January 2026.
| Start date | Saturday, 31 January 2026 |
|---|---|
| Result date | Saturday, 28 February 2026 |
| ISO format | 2026-02-28 |
| Calendar days from start | 28 |
- The day was shortened from 31 to 28 because that month is shorter.
Accuracy note: Results are calculated in your browser from the values you enter. Check anything that matters for legal, academic or financial purposes against the official source.
Frequently asked questions
What is 31 January plus one month?
It is the last day of February, which is 28 February in a common year and 29 February in a leap year. The calculator clamps the day to the end of the shorter month.
Does the calculator include the start date when adding days?
No. Adding 1 day to Monday gives Tuesday, so the start date itself is not counted. This matches how most people count days from a date.
How do I find the date 100 days from today?
Leave the start date on today, choose add, put 100 in the days field and keep the other fields at zero. The result shows the exact date and its weekday.
Are weeks always 7 days?
Yes. A week is converted to exactly 7 calendar days, so 4 weeks is 28 days, which is not the same as one calendar month.
Can I subtract years and months at the same time?
Yes. Choose subtract, enter the years, months, weeks and days you want to remove, and they are all removed from the start date, with years and months applied first.