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
This may seem a small issue to Microsoft HQ is the US but it's enough to put a lot of UK users off of using Oklanner. Which is a shame, as it is otherwise a good product