The point of the new since function is that the number of days always corresponds to the number of days that will show up in the report. In an inclusive report, a 6 day difference will result in 7 days being shown. That's why we subtract a day so that it remains the correct number when using inclusive reports. 

9/3 - 9/1 = 2 day difference -- 9/1 , 9/2 , 9/3 (in inclusive reports) = 3 days