I know this is probably old news to most. But wanted to show my findings just to verify any speculations:

4. Entry Period: The Contest begins July 21, 2011 at 12:01am Eastern Time (“EDT”) and ends October 12, 2011 at 11:59 pm EDT (the “Entry Period”). Entries that are submitted before or after the Entry Period will be disqualified. Sponsor’s computer will be the official timekeeping device for the Contest.
Can be seen here on the AMD giveaway contest rules!