Counting days between two dates without the off-by-one

Ask three people how many days there are between 1 March and 8 March and you may get two answers. Seven, if you count the gaps. Eight, if you count the days themselves. Both are defensible, both are used in real contracts, and the disagreement is the reason date arithmetic causes so many small disputes.

The two counts, plainly

• Exclusive, or difference. Subtract one date from the other. 8 March minus 1 March is 7. This is what software gives you by default and what you want when you are measuring elapsed time.
• Inclusive. Count both endpoints as days. 1 to 8 March inclusive is 8 days. This is what you want when you are counting days of attendance, days of hire, or nights someone is entitled to be paid for.

The rule of thumb: if you are counting events or days used, count inclusively. If you are measuring a duration, count exclusively. Say which one you used in the same sentence as the number.

Where the off-by-one costs money

Car hire and hotel bookings count nights, not days, which is a third convention again. A stay from Monday to Friday is four nights and five days, and an invoice for either can be correct depending on what was agreed. Notice periods and interest calculations usually run exclusively from a start date, but some contracts say "including the day of service", which changes the deadline by a day. Read the clause rather than assuming.

Leap years and the awkward end of February

Any span crossing 29 February gains a day, which is easy to forget when estimating in your head from month lengths. Adding "about six months" to 30 August lands on a date that does not exist in February at all, and different systems resolve that differently. If a span matters, count real dates rather than approximating in months.

Time zones do not change a date count

A day difference between two calendar dates is a count of calendar days, not of 24-hour periods, so it does not change when someone crosses a time zone. It also does not change on daylight saving weekends, even though one of those local days is 23 or 25 hours long. This is precisely why date arithmetic should be done in dates rather than by dividing a number of hours by 24.

State it the way that cannot be misread

The safest form in writing is to give both endpoints and the convention: "from 1 March to 8 March inclusive, 8 days". It is slightly longer than "8 days" and it removes the entire class of disagreement. When you need the number itself, the days between dates calculator gives the exclusive difference and the inclusive count side by side, so you can quote whichever your situation calls for and say which one it is.