how to store previous time member in a variable in bpc script

Hello experts,

I have a bpc script requirement where i hv to copy the data from the forecast stored in previous month to the current month..

I have to get the previous time member from the time period selected for this purpose.Suppose user has selected 2013.05, i have to get the forecast stored in comparison property of time period of 2013.04 in some variable.How can i achieve this?

Thanks in advance