*SELECT(%CMONTH%,"[PERIOD]","TIME","[ID]='%TIME_SET%' ")
*SELECT(%LYEAR%,"[YEAR]-1","TIME","[ID]='%TIME_SET%' ")
*XDIM_MEMBERSET TIME = %TIME_SET%,%LYEAR%.%CMONTH%
there are others ways to solve this but with this you dont need to use your property for last year
If you want to use your PY property you change the %LYEAR% select to:
*SELECT(%LYEAR%,"[PY]","TIME","[ID]='%TIME_SET%' ")
Brgds
Mattias