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 TeamPaul commented
We dont use Teams - we just use planner the web app.
I cannot change the date format from US to UK.
This is very annoying.