Hi Lan,
Your requirement is to change the due date according to your calendar date based on the request.
As advised by Mr.Jakub, You can directly change the due date in the contract - Conditions tab - select simulate - select your line item - and give the new due date as you requested. Even after the postings also you can do changes in due date for future conditions.
Attached screen shot FYI.
I hope that will solves your purpose if I am not wrong.
Regards,
Subbarao