MS Planner board doesn't follow my regional date format

I know this is an old thread but for anyone who was having this issue, click on your profile picture in the top bar of Edge, then the little gear icon. This will bring you to the settings page. On the left, click on languages.

The language bar should show up in the middle of the screen with "preferred languages" on it. Click on the three dots to the right of "English UK", or wherever your regional preference is, and click "display MS Edge in this language". If the option is greyed out due to work or IT policy, you can move English US down the order of preferences. I completely removed English US by clicking on the three dots and clicking remove. I then re-added it later and ensured it was at the bottom of the list. When I went back to MS Planner all my dates were now displaying in UK date format and a 9 month headache was finally resolved!