Normal Integer calculation and Date Time calculation are bit different.
For all types of DateTime operation using CDate , Convert.ToDateTime, DateTime.Parse, DateTime.ParseExact you need to specify the time.
When you don’t give time parameter, “00:00:00” will be added by default.
because of this behavior its always calculated as start datetime to end datetime not the start date to end date
You can use the attached workflow for your date calculation, and it will give your desired results
Date_Calculator.xaml (7.5 KB)