Set analisys rolling

[#1]

I have to create a set that allows you to filter previous 12 months.

I wrote this expression but it is not correct.

expression1:

sum({$<year= {$(=max(year))}, month= {"<=$(=max({<year={$(=max(year))}>} month))>$(=max({<year={$(=max(year))}>} month)-12)"}>} QTA)

ES:
if I am in 2010-03 period i want set filters from 2010-03 to 2009-03

Can you help ,please?