I’m assuming you are referring to the printed invoice and not the invoice maintenance screens. You can modify the printed invoice template but not the maintenance screens. If this is the printed invoice, which template are you using? You can find this in the SI Defaults settings.
I’m using the default invoice template – it does what I need it to, to be honest.
New Invoice – the date is yyyy-mm-dd
Listing Invoices – the date is yyyy-mm-dd
Printing Invoices – the date is yyyy-mm-dd
On the previous 2019 version – I have some invoices which printed 12-Oct-2020.
I have Looked in Settings > SI Defaults but I can’t see anything “date format” wise.
In the ini Files within Config, they are set to D-m-Y