Hi Rob Heathcote,
Thank you for querying in this forum.
According to your description, it seems like you have set date format as dd/mm/yyyy(UK date format) from OneDrive Settings>Reginal Settings. But the dates in your Microsoft Lists still display as mm/dd/yyyy(US data format).
May I know if my understanding of the scenario is right?
If yes, I understand the issue has affected your daily work and feel sorry about it. I'd like to help you.
For troubleshooting, may I confirm that when you create new Microsoft Lists, where do you store the new lists? In My Lists or SharePoint sites?
If My Lists, then the new list is saved in OneDrive for business, the date setting inherits from the OneDrive for business site regional settings.
From your description, it seems like you have set the locale as English(United Kingdom) in OneDrive Settings>Site Settings>Regional Settings. (If you haven't set it, please note that it may take 24 hours for the change to take effect)
If a SharePoint site, then the new list is saved in SharePoint site, the date setting inherits from the SharePoint site regional settings.
You may contact the SharePoint site admin, let him go to Site information>Site Settings>Site Administration>Regional settings>check if the locale is set to English(United Kingdom).
Besides, could you also confirm information below for troubleshooting? Thanks for your patience and effort.
1.Does the issue happen in a specific Microsoft list or all Microsoft lists?
2.Do other users in your Microsoft 365 organization have this issue?
We appreciate your understanding that sometimes the initial reply may not resolve the problem very soon. However, we can work together to narrow down and resolve the situation. Please kindly provide more information to let us work further.
Hope you are keeping safe and well.
Best regards,
Tina