Thanks for your reply.  Yesterday I didn't notice any due date value shown on the Sales Direct Invoice screen.  
Today I paid attention before inputting anything to the DI screen.  I FOUND there was a Delivery Details section at the lower part of the screen, and the due date value was shown.  It was defaulted the same as the invoice date: 31/Aug/2018 which was my year-end date.  
I changed both the due date and the invoice date to my desired date and then started to choose a customer.  All my customers are cash based, i.e. the payment term has been defined as cash only.  Once a customer had been chosen with no other details inputted, the Delivery Details section was immediately replaced with a Cash Payment section.  I completed the invoice input and placed it.  When I printed the invoice, the due date was back to 31/Aug/2018.  My due date input was not recorded at all.  This is kind of strange.
What's more, subsequent input of customer invoices would not show the Delivery Details section again unless I changed the payment of the customer to values other than Cash Only. I guess the Delivery Details is only shown for non-Cash Only customers.  Even though I changed the due date value, it would not be recorded for cash only customers.  But how come the due date was defaulted to the year-end value for my cash only customers?