Hi Josh,
Greetings! Thank you for posting to Microsoft Community.
Regional format settings in Excel affect dates, currency, formula delimiters and more. If you are using your organization account, the default regional format settings in Excel for the web are the site regional settings of the SharePoint site where the Excel workbook is hosted.
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- Click Home > Number Format > More number formats...
- Under Category, click Date, Time, or Special, and then select a locale from the Locale menu.
- Select the checkbox to set the chosen locale as your default regional format, and then click OK.
Reference:Change regional format settings in Excel for the web - Microsoft Support
Best Regards,
Snow Lu