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 Team
An error occurred while saving the commentJF commented
Same issue from Australia. More than anything it is just another example of really amateurish testing and market focus. Microsoft - you've got the money, get the competence.