In our case, the invoices are being sent with each "pay later" event registration. The default appears to be 10 days. The client wants to change the date to September 3 of this year. Do I need a new invoice template? Can I do that? Can I do this somewhere in the Event setup for a set due date for all registrations?
There is a setting in Administer > CiviContribute > CiviContribute Component Settings that lets you change from the standard 10 days (you will need to tick the "enable tax and invoicing box to see it). But that doesn't get a fixed date. If you aren't using the invoices for something else then you could edit the "Contributions - Invoice" template in the "system workflow messages" tab in Administer > Communications > Message Templates. Or if you do use the invoices for other things you could perhaps put some logic in the template to deal with the different cases. Hope that helps.