IIRC, last time I checked there was an issue with due dates. This was around 2 months ago so my memory is hazy... but here goes:
I was seeing one date (the correct date I had sent from the API) when viewing the 'Invoices' listing page. Then when clicking into the invoice, initially I would see my specific date I had set briefly before it would change to something else entirely.
I never tested what would get returned via the API but I would wager that it is whatever is on the listing page.
In the end I gave up attempting to change the date and just let MYOB do whatever it wanted and then at least I wasn't creating (or appearing to create) data discrepencies - even though the fault was at MYOB's end.
I understand this is probably not helpful to you, but at least know you are not alone with having issues with the due date.
