Should you count the start day and end day in a date range?
It depends on the calculation. To measure the raw difference between two dates, the start day is usually not counted. For an inclusive period, such as a rental from the 1st to the 3rd, both the start and end dates are counted, giving 3 days in total.
Explanation
The trap is that the same interval can be read in two different ways.
If you want the mathematical difference between two dates, you measure how many days have elapsed between the start and the end. Between the 1st and the 3rd, there are 2 days of difference: from the 1st to the 2nd, then from the 2nd to the 3rd.
But if you want to count a period that is used, booked or charged, you may need to include both the start day and the end day. In that case, a rental from the 1st to the 3rd covers the 1st, 2nd and 3rd: that is 3 days.
This is why date calculators often offer an option like “include end date” or “+1 day”. It does not change the calendar: it changes how the period is interpreted.
Formula / method
Inclusive duration = raw difference between dates + 1 day
Concrete example
From the 1st to the 3rd:
- raw difference between dates: 2 days;
- period with end date included: 3 days;
- booking from the 1st to the 3rd: often 3 counted days.
The right answer depends on what you want to calculate: a gap or a real period of use.
Common mistake
A common mistake is applying “+1 day” everywhere. It is useful for an inclusive period, but not always for a deadline expressed as a difference between two dates. For a contract, administrative rule or important deadline, check the exact wording.
Sources & methodology
This content follows Outilo's editorial guidelines.