Credit days count from contract date

Hi all,

One of our client is using credit payment terms like 30 days credit, 15 days credit and the counter of credit days start from contract creation date. It means that if the payment term is 15 days credit and contract creation date is 2/6/2009 then after 15 days, this payment become overdue.

My query is this how can we do this in SAP as in standard SAP , credit days counter starts from Invoice date.

Please respond.

Best Regards,

AI.