DUE DATE FOR CREDIT MEMO

Hello,

I have a question about credit memo -due date.

Using MIR7, MIRO for credit memo the due date for net payment is the same with baseline date.

e.g. document date:18/07/2007

posting date:18/07/2007

baseline date:18/07/2007 terms of payment are 30 days

the system issue due date : 18/07/2007 but correct is 18/08/2007

What can I do to calculate due date correct for credit memo ? (baseline date + terms of payment) ?

Thanks.