How it is worked out
resulting date = starting date ± number of days
Why it is done this way
Adding days to a date sounds trivial and is not, because months have different lengths and February changes size. The reliable method is to convert the date to a day count, add, and convert back — which is exactly what a calendar library does and what a person doing it by hand keeps getting wrong.
Step by step
- Start from your date
- Add the days, carrying over month ends as you go
- Check the month length: 30, 31, or 28 or 29 for February
- Use a negative number to go backwards
What is worth knowing
The working-day answer is usually the one that matters, because deadlines, notice periods and delivery times are rarely counted in calendar days. Note that this calculator only skips weekends: public holidays vary by country and even by region, so it cannot know yours. For anything contractual, check whether the deadline is stated in calendar days or working days — the two can differ by a week and a half over a month, and the difference is not academic when it is a deadline.