What this calculator does
Adding days to a date is unambiguous. Adding months is not, because months differ in length and there is no universally agreed answer to what one month after 31 January should be. This calculator resolves it by letting the month overflow: 31 January plus one month gives 3 March, because February 2026 has 28 days and the extra three roll into March.
That is worth knowing before relying on it. Other tools clamp to the last day of the target month instead, giving 28 February for the same question, and contracts sometimes specify which behaviour applies. Where the distinction matters, adding days rather than months removes the ambiguity entirely.
The formula
The years, months, weeks and days are applied in that order to the starting date, with weeks converted to days first. Months are added by incrementing the month number and keeping the day of the month, so a day number larger than the target month carries forward into the following month rather than being clamped back. The result is reported with its weekday, ISO format and ISO week number.
| Term | Meaning |
|---|---|
| Overflow | What happens when the day of the month does not exist in the target month: the surplus days roll into the next month. |
| Clamping | The alternative convention, where the result is pulled back to the last valid day of the target month. Not used here. |
| ISO format | The unambiguous YYYY-MM-DD written form, which avoids the day and month order confusion between regions. |
The inputs explained
| Field | What to enter |
|---|---|
| Start date | The date to start from. |
| Direction | Whether to add the period to the start date or subtract it. |
| Years | Years to apply. |
| Months | Months to apply. Read the note above about month-end behaviour before relying on this for a contractual date. |
| Weeks | Weeks to apply, converted to days internally. |
| Days | Days to apply. |
When to use it
Finding a deadline a set period away
Notice periods, warranty expiries and review dates are usually specified as a period rather than a date, and this converts one to the other in a single step.
Working backwards from a fixed date
Switching to subtract answers when something has to start in order to finish on time, which is the more common planning direction.
Checking which weekday a future date falls on
The weekday of the result is often the deciding detail, since a deadline landing on a Saturday usually moves in practice even when the contract does not say so.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What happens when you add months to 31 January?
The month-end case, added to a fixed start date of 31 January 2026.
| Months added | Resulting date | Day of the week | Days from the start date |
|---|---|---|---|
| 1 month | 3 Mar 2026 | Tuesday | 31 |
| 2 months | 31 Mar 2026 | Tuesday | 59 |
| 3 months | 1 May 2026 | Friday | 90 |
| 6 months | 31 Jul 2026 | Friday | 181 |
| 12 months | 31 Jan 2027 | Sunday | 365 |
Questions
What is one month after 31 January?
This calculator gives 3 March in a common year, by letting the surplus days overflow. Many other tools give 28 February by clamping to the end of the month. Neither is wrong in general, but they are different, so check which convention applies if a legal or contractual date depends on it.
In what order are the units applied?
Years, then months, then weeks and days together. The order matters at month boundaries: applying a month before a day can give a different result from applying the day first. Using days alone avoids the question.
How do I add business days rather than calendar days?
This calculator works in calendar days only. For a count that skips weekends, use the working days calculator, which counts weekdays between two dates rather than projecting a date forward.
What does the week number mean?
It is the ISO week number, where week 1 is the week containing the first Thursday of the year and weeks run Monday to Sunday. It is widely used in European business and manufacturing scheduling, and does not always agree with other week-numbering conventions.
For the gap between two dates you already have, see the days between dates calculator. For a working-day count that skips weekends, see the working days calculator.