Add or Subtract Days Calculator
Includes the end-of-month rule, which is where date arithmetic quietly goes wrong.
The end-of-month problem
Adding days is unambiguous. Adding months is not, because months have different lengths and there is no universally correct answer to “what is one month after 31 January?”
The convention almost all software uses, and the one applied here, is clamping: if the target day does not exist in the destination month, use the last day of that month. So 31 January plus one month is 28 February (or 29 in a leap year).
Clamping is not reversible. 31 January + 1 month = 28 February, but 28 February − 1 month = 28 January, not 31. If you are chaining month arithmetic, always work from the original date rather than stepping repeatedly, or the drift accumulates.
The same applies to 29 February in a leap year: add one year and you get 28 February.
Days, months and legal deadlines
Legal and commercial deadlines are written in several incompatible ways, and the wording matters:
- “Within 30 days” — calendar days, usually excluding the start date.
- “Within one month” — the corresponding date in the next month, with clamping. This is a different length depending on the month.
- “30 working days” — roughly six weeks. Use our working days calculator.
- “Clear days” — excluding both the first and last day, common in court procedure.
Where a deadline carries real consequences, count it out on a calendar as well as calculating it. A deadline falling on a weekend or public holiday is often extended to the next working day, but not always — that depends on the governing rules.
Common uses
Typical periods people need to calculate:
| Period | Typical context |
|---|---|
| 14 days | Consumer cancellation rights in the EU and UK |
| 28 days | Notice periods, some rental agreements |
| 90 days | Probation, visa-free stays, warranty windows |
| 6 months | Passport validity requirements for travel |
| 1 year | Warranties, insurance renewal, anniversaries |
| 180 days | Schengen 90/180 rolling calculation |
Note that a rolling window like the Schengen 90-in-180 rule is not a simple date addition — it requires counting days present within a moving window, which is a different calculation entirely.
Frequently asked questions
Is one month always 30 days?
No. This calculator's 'months' option moves to the same date in a later month, which is between 28 and 31 days. If you specifically need 30 days, select days and enter 30 — the two give different answers for most starting dates.
What happens with 29 February?
Adding a year to 29 February 2028 gives 28 February 2029, because the target date does not exist. Some legal systems treat 1 March as the correct answer instead, so check the applicable rule where it matters.
Can I subtract instead of add?
Yes — enter a negative number. Subtracting 90 days from a date finds when a 90-day period began.