As an update, since we’ve still been seeing issues around date formatting, so we’re pursuing another fix that should help: If you’re using Planner within Teams, we will default to using the same language/local as Teams.
If you’re seeing this issue persist outside of Teams and your language/locale is set properly for your Office account, please let us know in the comments below!
The Planner TeamPeter Sv commented
From what I can see, this i still not fixed and there's no way to work around it. At least I can now stop searching for what I thought I'd overlooked. This is such a silly problem to have. Crazy and annoying