Originally Posted by US CBP

For instance, if you come to the U.S. on July 10, 2005 on a B2 Visitor Visa, you may go to Canada and/or Mexico on or after November 10, and reenter the U.S. any time up until Dececember 10. But because the six month period is up on December 10, 2005, you will also have to depart from the U.S. on that same day to avoid being an "overstay" (unless you applied for an extension of stay.)