What is your current time member? For this to work correctly you will need to be referring to the day not the month.
For instance:
current date = 11/5/03 = [Time].currentmember
ParallelPeriod([Time].[Year], 1) = 11/5/02
MTD(ParallelPeriod([Time].[Year],1) should return the set - 11/1/02-11/5/02
The sum of this set should return the value you are looking for. The only way this will not work is if your Time.currentmember is not the last day of your set. In that case you may need to explicitly state the Time member you are working with.
Hope that helps.
Steve Hughes
Magenic Technologies