Payment term definition

Hi guys:

I have one new payment term to be defined in OBB8.

Payment term due date on 31 days after month end.

For example, if invoice posted on 11/08/2009, due date should be 01/10/2009. But when i specify fixed date 31,additional months 1, the due date will show 30/09/2009.

Anyone has idea on this?

Forrest