datetime requirement of the date is in this format 'YYYY-MM-DD HH:MM:SS'
i.e. "2012-04-18 00:00:00" or similar.
You can still display the date in the format 04-18-2012 by using date_format (date, "%m-%e-%Y") in your sql query.
You can also enter it as 20120518 but not 05182012