How the new date is found
For days and weeks, the calculator counts forward or back from the start date. The start date itself isn't counted, so 1 day after Monday is Tuesday.
For months and years, it keeps the same day of the month. When that day doesn't exist, like 31 January plus one month, it uses the last day of the month instead: 28 February, or 29 in a leap year.
For working days, it steps one day at a time and skips your chosen weekend days. Public holidays aren't skipped, because they differ by country and employer.
Questions people ask
Is the start date counted?
No. Adding 1 day to 10 March gives 11 March. This matches how deadlines like “within 30 days” are usually counted, but check the exact wording of any contract you're working from.
What happens when I add a month to 31 January?
There's no 31 February, so the result is the last day of February: the 28th, or the 29th in a leap year.
What's the ISO week number?
It's the week of the year under the ISO 8601 standard, where weeks start on Monday and week 1 contains the year's first Thursday. It's the week numbering used in most of Europe and in many business calendars.
Can I skip public holidays?
Not automatically, because holidays differ by country and employer. Add one working day for each holiday that falls in the period.