Select your two date columsn on the date view page and then select Pivot from the drop down menu that appears when you right click on the selected column headers. You will then get three columns in the pivot output -
Column 1 will have the letters - a, b, c,d, e,a, b,c,d,e
Column 2 will have those two dates September, 2015 and September, 2016
Column 3 will have pivot field values - the values from your 2 columns.
The issue you have is that those second column values are going to be positive so we will need to take care of that.
i do that with this calculation:
if [Pivot Field Names]="Sep-15" then [Pivot Field Values] else -[Pivot Field Values] END
The result workbook shows this in more detail with sheet 3 showing the result desired.
I hope that helps. Please note that the attached workbook is in v10.1.
column subtraction excel.twbx 43.5 KB