Based on your expression for current, previous would be very similar... Note that the below will 'roll back' from a Period 01 to Period 012.
IIf(DatePart("mm", GETDATE()) == 1 , "012", "0" + (DT_WSTR,30)DatePart("mm", GETDATE()))
You don't mention it but make sure you account for year 🙂
Steve.