Try it
Change the dates and tick the box to see which days are counted.
Exclusive counting: the days after the start
Subtracting one date from another gives the number of days that pass between them. From Monday to Friday, four days pass: Tuesday, Wednesday, Thursday and Friday. This is how our calculators count by default, and it is how spreadsheets work when you subtract two dates.
Use it for durations: how long until a deadline, how many nights in a stay, how old something is in days.
Inclusive counting: both dates
If both the first and the last day are part of what you are counting, add one. A conference from Monday to Friday lasts 5 days; a course that runs from the 1st to the 10th has 10 class days.
Use it for anything where both ends are “used”: events, rentals charged per day, schedules and rosters.
A quick test
Ask what happens when both dates are the same. If the answer should be 0 (no time has passed), count exclusively. If it should be 1 (one day is used), count inclusively.
Legal and contractual deadlines often have their own counting rules, such as excluding the day of the event or moving a deadline that falls on a weekend. Check the rule that applies to you. See also why date calculators disagree.