Hi Syed,
If you are not able to achieve the solution by configuration, then I can suggest you a technical solution. If you trigger the Action Profiles, then you can create an implementation of the BadI : EXEC_METHODCALL_PPF. You can then call the Order Read API to get the start date. You can then calculate the end date and assign it to the required document using the ORDER Maintain API.
You can also trigger some additional logic if there is a requirement for that.
I hope this helps.
Thanks,
Samantak.