Counting days between dates
The gap is counted from the start date up to but not including the end date, which is the standard convention for durations. From 1 March to 8 March is seven days. If you need the count to include both endpoints — for a hotel stay or a holiday allowance, say — add one.
Working days
The working-day count treats Monday to Friday as working days and excludes Saturday and Sunday. It does not know about public holidays, which differ by country and sometimes by region, so subtract those yourself for contractual deadlines.
Adding months is not the same as adding days
Adding one month to 31 January cannot land on 31 February. Following the JavaScript date convention, the result rolls into the next month — worth knowing when a contract says "one month from" an end-of-month date. Adding a fixed number of days avoids the ambiguity entirely, which is why legal drafting often prefers it.
Useful patterns
- Deadlines: add 30, 60 or 90 days to an invoice date.
- Notice periods: subtract the notice length from a leaving date to find the last day to hand in.
- Project planning: use the working-day count to size a schedule realistically.
- Countdowns: set today as the start and the event as the end.
- Ages: the age calculator gives an exact age from a date of birth.