You need to set up terms. List menu > Customer & Vendor Profile lists, Terms.

Then, you Assign that to customers as their default. That way, your standard of Net 30 might not apply to a customer that contracts as Net 60.

Then, when you invoice, the system Calculates and sets the due date for you.

There is no Global default. You either manually change the due date as you create invoices; or, you manually select from the Terms dropdown on that invoice, to let the program set the due date; or, you assign a Customer their default terms, and the program does all of this for you.