You can have a look at the format/parse date time functions that Mendix provides;
https://docs.mendix.com/refguide/parse-and-format-date-function-calls
You can, for instance, format the datetime into a string before passing it on to your Excel exporter;
formatDateTime($DateTimeAttribute, ‘yyyy-MM-dd’), which results in a string containing the datetime in yyyy-MM-dd format.
You could parse the datetime you want to export, and use this in the export functionality.