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 B commented
All works fine for me until I use the 'open in planner function' from the ToDo app. I can see no locale settings in ToDo but it seems to have the correct date formats for the UK within the app. The issue comes about on this occasion because the way the URL is formatted by ToDo. It creates a URL with EN-US in it. While you allow Planner date setting to be governed by a URL you will always need developers to build additional complexity into all apps that form links to Planner to ensure the link is formatted correctly. Surely the answer for planner and all MS apps is to take the locale settings from the local machine? As a user, I don't want to have to adjust locale settings for each MS app that I use. Especially as the MS user interface is not consistent across apps and users are forced to hunt around in each app to find out where the apps developers have decided to hide them.