It always works fine in my tests. There is no magic number, all date cells have date format. However, it is possible that it depends on the system locale, date format or software version (I tested in LibreOffice and Google Sheets, but not in Excel).
Please share your project file, log file (Help > View log) and let us know in what software do you open the result and is it the same if you open it in other software which supports Excel format.
Please share your project file, log file (Help > View log) and let us know in what software do you open the result and is it the same if you open it in other software which supports Excel format.