You are able to specify payment terms in Settings -> Invoice, as below:

By setting your payments terms, the Due Date field on the Invoice Details screen will automatically be populated with the invoice date + payment terms. So, for example if you set payment terms to 30 days and you raise an invoice on 13/11/2018 the due date will be displayed as 13/12/2018. If you wish to over-ride the due date, you can just amend it here.

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