Date as variable in a set analysis

[Anonymous]

Hello.

I'd like to compare the sales between a certain date (YYYY-MM) and the same period but a year before.

This certain date is a manually typed variable named MonthA.

I've been able to graph the sales of that exact month with the following expression, even thought it says 'Error in expression':

sum({$<time.MonthYear={'$(=date(date#(MonthA,'YYYY-MM'),'YYYY-MM'))'}>} Sales).

I've tried using two manually typed variables, MonthA and MonthB and it works but I'd like to accomplish the same with an expression.

sum({$<time.MonthYear={'$(MonthA)','$(MonthB)'}>} Sales)

I've tried using addyears(date(date#(MonthA,'YYYY-MM'),'YYYY-MM')),-1) but it doesn't work. Could you help me out?

Thanks a lot.