Hello Sesha,
Its going to be a tough one if you want to calculate due date based on invoice date and payment term. Instead I wud recommend you another work around. 0fi_ar data source automatically calculates net due date at the extractor level itself for all the accounting documents. Do a look up in your reporting cube to bring in the net due date of the corresponding invoice document for all the AR document.
Let me know your feedback
Regards
Gajesh