TERM DATE BASIS - customization

hello Gurus

we have come across a requirement to calculate due date based on a fixed Terms date.  (system only gives option of having INVOICE DATE, SYSDATE, GOODS RECEIVED DATE, INVOICE RECEIVED DATE)

Example:  I want to calculate due date based on some standard terms date basis (say 1st of next month)

say supplier terms are NET 15 days.  If i create a invoice today (7-Nov).  It should populate due date as 16-Dec (01-Dec + 15 days)

any suggestions to customize ?

Thanks