Why adding months is not simple
Adding days is unambiguous — a day is always a day. Adding months is not, because months have different lengths, and the answer depends on a convention rather than arithmetic.
What is 31 January plus one month? February has no 31st. The two common answers are 28 February (clamp to the end of the month) and 3 March (overflow into the next). Most software, including this calculator, clamps, because that is what people mean and what most legal and financial conventions use.
| Start | Plus 1 month | Why |
|---|---|---|
| 15 January | 15 February | Straightforward |
| 31 January | 28 February | February has no 31st — clamped |
| 31 January (leap year) | 29 February | Clamped to that year's last day |
| 31 March | 30 April | April has 30 days |
| 30 January | 28 February | Also clamped |
A worked example
Note that "30 days" and "one month" are different instructions. One month from 15 March is 15 April — a day later than 30 days, because March has 31 days.
Business day arithmetic
Ticking "business days only" skips Saturdays and Sundays when counting the days field. Ten business days from a Monday lands on the Monday a fortnight later, not on the Thursday of the same week.
This matters for anything with a working-day deadline: payment terms, notice periods, statutory response windows and court filings.
To measure the gap between two known dates rather than project forward, use the days between dates calculator. For exact ages, the age calculator handles birthdays and leap years.
Frequently asked questions
How do I add days to a date?
Enter the start date, put the number in the days field, and the result appears immediately. The calculator handles month lengths and leap years automatically.
You can combine units — 1 year, 2 months and 10 days are applied in that order.
What happens when I add a month to 31 January?
You get 28 February, or 29 February in a leap year. February has no 31st, so the date clamps to the last day of the month.
This is the convention used by most software and by most legal and financial contracts.
Does the calculator account for leap years?
Yes. Adding 365 days to 1 January 2024 gives 31 December 2024, because 2024 is a leap year and contains 366 days.
This is why counting in days and counting in years can give different answers across a leap year boundary.
How are business days counted?
Saturdays and Sundays are skipped. Ten business days from a Friday lands on the Friday two weeks later.
Public holidays are not excluded, since they differ by jurisdiction.