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 TeamGilles Couzin supported this idea ·Gilles Couzin commented
There is a discrepancy in the date format between looking at a plan from the Planner app itself or from the Planner web part embedded on a SharePoint site page.
The former shows the correct UK date format (i.e. dd/mm/yyyy) while the latter shows the same dates in the US format (i.e. mm/dd/yyyy). For example, a Start / Due date of 1st September 2019 shows as '01/09/2019' in the Planner app and as '09/01/2019' in the SharePoint Planner web part.
Regional settings are set correctly to UK, both in Office Profile and SharePoint, so this is a bug in the Planner web part, which makes it unusable outside the US.