Payment Terms, Invoice Due Date

Hi,

I've got to set up a new payment term in our system. The requirement for this payment term is:

A) The invoice due date is either, whichever is the later of:

i) The 12th day after the day on which the invoice document was deemed to be received : and

ii) The 20th day after the last day of the billing period to which the invoice document relates.

I don't know how to configure this, can anybody help?

John