The Due box on the invoice form offers On receipt, 7, 14, 21 or 30 days, a custom date, or no due date at all. An invoice due on the day it is issued prints "On receipt" rather than a date.
Set your normal terms once in Settings, under Business details, and every new invoice starts with them - if all your work is payable on receipt, you never touch the box again. You can still change it on any single invoice.
The due date is what the reminders run off: on the Growth plan, DocMason chases an unpaid invoice the day after it falls due and again a week later.