Most likely because the existing dates in your flat-file don't conform to the mySQL DATE format which is YYYY-MM-DD

If you only have a few records in your flat-file it is simple enough to manually change them to the proper format before you import into mySQL table. If not then you'll have to pull your flat-file into something like Access, reformat the date field, then export back out to a flat-file, then import into mySQL.

HTH

easy does it