Net due date calculation

I need to calculate the net due date for the billing invoices which are net released to accounting yet.

1. Is there any function which can calculate the net due date with taking into consideration billing date, terms of payment, "additional value days" (VBRK - VALTG) and "fixed value date" (VBRK - VALDT) ?

2. Is there any function which can give out the net due date for an SD invoice which is not released to accounting?