Counting every day
With days in total picked, the start date counts and the end date does not, which gives the usual "difference" between two dates: from 1 March to 31 March is 30 days. Tick Count the end date under Options to count both ends, which gives 31.
Counting working days instead
Change days in total to working days in the sentence. Weekends and the public holidays of the place you pick are then skipped, and both dates count unless you untick them.