assuming that you allow the user to select the month:
1. save the selected month in a variable, compute for the prior month in another variable. then use these two variables in your set analysis
2. search for the AS OF TABLE this will be a mod to your data model such that when the user selects a month, you can use another field to point to the current month or prior month and use that field in your set analysis. the advantage of this is you delegated the calculation to your data model which will be more efficient for large data sets