The spreadsheet and work books were saved over from excel to trial Libreoffice, which so far I have been very impressed by.
First, there is no “trial LibreOffice”. There are only full LibreOffice releases on the download page.
Second, what exactly do you mean by “returns MM-DD-YY”? Already existing dates are displayed such instead of DD-MM-YY? Or when entering a date the displayed date then has day and month swapped? Or …?
When I say “trial Libreoffice” I mean “give-it-a-go” as an alternative to microsoft office 365 which I have been using since dot. In anycase, a bit more reading and I came across what I was looking for (see answer). It was a case of changing the default language across the entire workbook rather than within a selected cell range (>format cells) which seemed to find it difficult to decipher between mm/dd and dd/mm until I changed the default language to Australia.
With a bit more reading and getting to know Calcs and LibreOffice I found my answer in Setting up text language defaults. For anyone who is looking for a quick go-to, you will see “Text language” tab at the very bottom of the interface with your workbook open. Right click > More… >Options-Libreofficecalc-Defaults >Language settings >Languages
This will provide the means to alter the default (US) to what ever language and currency denomination one wishes to use. Once the changes are made you will be prompted to save and close for the new defaults to take affect.
The default locale (language+country) is chosen according to the system settings. So if that was en-US then probably the system user profile is using en-US (or in rare cases some locale that LibreOffice doesn’t know). If that was en-AU instead then everything should had worked as expected.