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Hi AdminEPMA,
Thanks for posting in our forum. We are glad to help you.
Based on my test, the behavior might be related to the Time Zone and region. The file you uploaded to Teams page is stored in SharePoint Online group site, when you open it in Excel Online, it will use the Time Zone/region from that group site. If group site's time zone/region is different from your computer. It may display a different value in Excel Online.
On your SharePoint online Teams page>File>Open in SharePoint Online>you will be redirected to the SharePoint Online group site page>click Setting gear on the top right>Site Information>View all site settings>Regional settings>Check the Time Zone and locale. If you are the team owner and all other users are in the same time zone and locale as you. Change the value to be the same as your computer. Check the results.
For your reference: Modified time is not displayed correctly for a SharePoint Online list item
Let us know if you need additional assistance.
Best Regards,
Waqas Muhammad