Thank you, so much for your input Gino. I’ve found the same thing. Even adding new date values works as expected and keeps the underlying dates in the correct format. It’s only the display as you say. I have also noticed a similar issue in the projects plugin. The date displayed in cards on the board is also wrong, but the actual data is correct.
It’s not a bug : How date and date & time are displayed within Properties relies on your device/OS settings (on Mac you can change those in Settings → Language & Region. No idea how to do so on Windows/Linux though).
You might want to add your vote and/or voice to this FR :