Microsoft Excel supports 2 types of date. Date is the number of days starting from 1/1/1900 or 1/1/1904. The first one is the default the second one has been added cause Macintosh supports it. The Microsoft library supports both of them and you can choose programmatically which one you prefer (https://docs.microsoft.com/en-us/dotnet/api/microsoft.office.interop.excel._workbook.date1904?view=excel-pia). Expose such an option in SpreadProcessing as well.