When you want to set up or change your invoice due date
Initially, you can set up your default amount of days that the invoices are due after sending.
To do this: Go to 'Settings' > 'Company' and 'Information':
Once on this page, you can change the number of days the invoice is due after sending in 'Invoice Due (Days)' at the bottom on the right side. Don't forget to 'Save' your changes.
You can override this default setting when you come around to sending your invoices.
Go to 'Billing' > 'Invoices' and select the invoice/s you want to send.
Under 'Group Actions' press the 'Approve' button.
Just before these are sent, you can manually change the due date. You will get a pop up with the default date in it that you are able to alter. When you have finished, press 'Approve'.
This due date will appear on the invoice for the customer.