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Thank you for posting your question in the Microsoft Q&A forum.
Regarding your issue, unfortunately, Microsoft 365 does not provide a built-in option in the Microsoft 365 admin center to globally enforce regional settings (such as date format) across the entire tenant. The reason is that these settings are tied to individual user profiles and SharePoint/OneDrive site collections, not managed by a single global toggle.
But if you still want to standardize settings across OneDrive and SharePoint sites, you can try using PowerShell if you're comfortable with it. For more details, please refer to: Onedrive regional settings - Microsoft Q&A
While it's frustrating to rely on users, this is the easiest way to ensure the date format is correct for individual web sessions and new accounts. For more details, please refer to: Change your personal language and region settings.
Also, I suggest telling users to check/change Excel for the web’s locale settings when they open documents, which helps avoid date-format issues during their sessions. For more details, please refer to: Change regional format settings in Excel for the web.
Hope this clears things up and I apologize for the inconvenience this issue has caused and thank you for your understanding.
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