An invoice has a creation date and a due date.
The due date is defined by the individual customer's terms of trade:
Note: Changing Terms of Trade, will only affect new invoices. To change an existing invoice you need to edit the Invoice header.
So if an invoice is created on the 28/05/2018 and Terms of Trade are the 20th of the month, then the due date will be 20/06/2018.
When the statement run is processed (usually on the last day of the month) invoices due will be listed on the statement and due payment on the 20th of the month.
Warning
If a customer does not pay the invoice on or by the 20th, the invoice is technically overdue.
If an overdue notice procedure was executed on the 15th of the month, any invoice not paid would be identified as requiring an overdue notice.