Set invoice due date

Hi
I noticed in the email Dynamic Parameters there is a %due_date% field which says “is replaced by the related invoice’s due date (if set)”. How do I set the invoice due date? i cannot see anyway to set it on the new invoice screen or the invoice edit screen.
I am only using manual payments.

if oi can set it, is there a way to report those unpaid invoices that have exceeded the payment due date?

Many thanks

Dave