4) Entry date is the data when you enter the invoice in SAP. There are companies which establish this rule: if the vendor sends the invoice late, I want to calculate the due date by the time i received this invoice (I have to post it in the system in the same day). Hope it help...
In your case, you can loop 20 times (Loop BSEG table as you have 20 line items) and add 30 days to due date of previous line. So, month and year are dynamic for you. http://scn.sap.com/community/erp/financials/blog/2013/09/30/fixed-day-payment-terms--the-magic-behin... Hope ...