Use a STR([cal_date_nkey]), that should convert it to string ... however, be careful with your comparison, you want to make sure your right side respects the format of your date field.
can you save as packaged workbook? there is no TDE attached on a twb.
Here you go. Click on the link and it will take you to the workbook. The workbook size is huge thats why I am sharing by public.