How to stop Excel Reader from treating numbers as dates?

When I use the Excel reader, it is automatically converting anything that looks like an Excel date serial number to a date, even when I change the column to string in the Transformation tab. I have tried changing the format in the Excel file to general, to number, and even adding something like .123 to the number so it looks like a number. How can I get the Excel Reader to treat these as a number (or preferably, a string)?