Add or subtract days from a date
Deadlines are usually expressed as a period from a date, and working days give a different answer from calendar days. This handles both, and clamps month arithmetic properly at month ends.
- In ISO format
- 2026-11-20
- Calendar days apart
- 30
- Working days apart
- 22
- ISO week number
- 47
- Day of the year
- 324
- Quarter
- 4
The formula
Days and weeks shift the date directly. Months and years use month arithmetic with the day clamped to the length of the target month, so 31 January plus one month is 28 or 29 February, not 2 March. Working days step over Saturdays and Sundays one day at a time.
Worked example
30 calendar days after 20 October 2026 is 19 November. 30 working days after it is 1 December — a difference of twelve days.
How to use it
- 1Read your contract wording carefully: 'within 30 days' and 'within 30 working days' are nearly a fortnight apart.
- 2Working days here exclude weekends only. Add public holidays yourself.
- 3The ISO week number is the one European business calendars use, where week 1 contains the first Thursday of the year.
Questions people ask
What happens at the end of a month?
The day is clamped. 31 January plus one month is the last day of February, because there is no 31 February. Adding a month twice from 31 January gives 31 March, not 28 March.
Why is the ISO week number sometimes 53?
Because 52 weeks is 364 days. Years that start or end on the right weekday get a 53rd ISO week, which happens roughly every five to six years.
Last reviewed: 2026-10-20