DIM_TIME: year->month

1. If CurrentMember of DIM_TIME is in the past (less then system month),
then roll M_A up only. I think I got that part:

IIF(FORMAT([DIM_TIME].currentmember.Properties("Mth"), "YYYYMM") <
FORMAT(NOW(), "YYYYMM")
,
Sum(PeriodsToDate([DIM_TIME].[(All)]), ValidMeasure([Measures].[M_A]))
,
the else part is the one I need hlep!!!)

2. If CurrentMember of DIM_TIME is this month (system month) or in the
future, I need to add roll up M_A (from the past) and add M_B for this month
and/or future:

the else part I need help:

Sum(PeridsToDate(until last month, 4/06 in this case) +
Sum(PeridsToDate(from 5/06, this month, to CurrentMember)

Thanks for any help.
MJ