Terms of payment - from GRN date

Dear All,

I have a payment terms of 30 Days.

Currently system calculates  30days from the date of invoice.

But I want to calculate 30 days from the date of GRN and not from Invoice.

Means if I have made GRN on 02.03.2013 and purchase invoice is booked on 10.03.2013 then system should put due date as 02.04.2013.

Kindly guide which set us is required to be done for this.?!

Thanks,

Jainal