Due Dates on Invoices

I haven't been able to find out how to include a due date on the printed out version of invoices. Also, it would be nice if you could put in a time frame for invoice due dates. For instance, if I created an invoice on April 1st and I wanted the due date to be 10 days after the creation of an invoice, the system would automatically put in April 11th for the due date which would print out on the invoice.
Amanda Carter