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 TeamAnonymous commented
Agree with other comment. Setting the language to English-Australian in Teams fixes Planner within a Teams tab, but when you launch out to a browser it defaults to en-US and the date format is therefore back to US.