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Hi, the date display may be coming from the iPhone/iOS regional settings or from the way Excel/OneDrive is rendering the workbook on iOS, even if your Microsoft and OneDrive accounts are set to Denmark. Since the file shows dd/mm/yyyy on Windows but mm/dd/yyyy on iPhone, please first check iPhone Settings > General > Language & Region and confirm that Region is set to Denmark.
In the workbook, you can also try selecting the date cells on the Windows PC and applying a fixed/custom date format such as dd/mm/yyyy instead of a region-dependent date format. Microsoft notes that Excel date formats can depend on regional settings, while custom date formats can be used when you want a specific display format.
https://support.microsoft.com/Excel/format-a-date-the-way-you-want-in-excel