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 TeamRichard Griffin commented
Latest update from Microsoft - cut and paste so please don't blame me for the appalling spelling and grammar.
This is Sagar from the Office 365 Support.
Issue: Teams date format wrong
We have checked that this particular feature in Teams desktop client has not yet rolled out:
You can follow the link shown below:
In traditional American usage, dates are written in the month–day–year order.So its taking bydefault this format.But for webportal its not.
Thank you for being a part of Microsoft Family.
Thanks & Regards,
Microsoft O365 Support Team