How can I calculate using the default script by using data from the previous month?

Hello Experts,

I'm trying to calculate  formulas with  data of the previous month via  default skript..

So I tried this way:

[#ACCOUNT1] = ([ACCOUNT1],[TIME].PrevMember) - [ACCOUNT2]

So for example: Account1 in 2013.02= Account1(2013.01) - Account2(2013.02)

But it doesn't come up with any value...

Can you help?

Many thanks in advance!