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....