"Article 24 Withdrawal of the United States Forces from Iraq ... 1. All the United States Forces shall withdraw from all Iraqi territory no later than December 31, 2011.
2. All United States combat forces shall withdraw from Iraqi cities, villages, and localities no later than the time at which Iraqi Security Forces assume full responsibility for security in an Iraqi province, provided that such withdrawal is completed no later than June 30, 2009.
..."
There is a lot more - read it for yourself if you'd like. There is nothing there that I can see that requires the US to remain until 12/31/11.