Payment terms (12.1.1)

We want to set up payment terms in AP where some invoices with invoice_date in eg. November 2012 get due_date = 30th of November + 15 days, others get due_date = 30th of November + 30 days, while a third group gets 30th of November + 45 days. Invoices with invoice_date in December gets due_date = 31st of December + 15 days, due_date = 31st of December + 30 days and due_date = 31st of December + 45 days etc.

As far as we have seen this functionality is possible to set up in the Special Calender form in AP. But there is one problem, if you have say 5 different categories like above you have to set up a special calender per category and per month. So you have to set up 12x5 = 60 special calenders per year which is time consuming and you also have to remember to create new calenders each year.