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 commentKhalid sarab commented
Still not working, could we have an update on this please? only way we can achieve this at the moment is giving users a URL to planner with 'en-GB', but this is not ideal.