Which exit date do I trust? I-94? or 30-day Grace Period?

[soulands]

I have an employee working for me on the rare Q-1 Cultural Exchange visa. The official final date of employment (stated on all visa application documents) is May 29, 2010. The Q-1 visa allows for a 30-day grace period beyond the final date of employment. But the I-94 card was dated as May 29, 2010. Does that mean the employee MUST be out of the USA by May 29? Or can they stay 30 days beyond that? I have read conflicting information on the uscis website about which date is the offical "be-out-of-the-usa" date... the date written on the I-94 by the point-of-entry officer, or 30 days beyond the final date of the employment program?