Due Date calculation

Hi ,

Getting one problem,
One form is there VendEditInvoice  , in that form DueDate is automatically calculated when we give invoice Date (DocumentDate)
but my requirement is that , it should calculate the due Date as per the PostingDate , not on DocumentDate and i need also to disable the Duedate calculation as for DocumentDate , it should only calculate dueDate for PostingDate.
So please tell me where i need to change the code , i have searched the methods at datasource level , but so many methods are there ,
So please tell
Thanks