As a reminder, Qlik interpret dates (coming from your Excel file) like numerical values! So, you need to convert your headers (directly in your Excel file) into text. It's simple, just add a single quote at the start for each text (inside each cell).
Hi @MPH , I had a similar problem were I was not able to modify the column names to date format For values inside fields its possible to use date() function to format them as data , but I didn't find a solution to do so for column headers. We can manually rename them in data manager window , if its a one time thing