It’s not an “arbitrary date,” it’s a “general time horizon.” From the White House a few minutes ago:

President Bush and Prime Minister Maliki spoke yesterday in their regularly scheduled secure video conference, about a range of matters including the improving security situation and the performance of Iraqi Security Forces across Iraq, from Basra, to Maysan, Baghdad and Sadr City, and Mosul…
In the context of these improving political, economic, and security conditions, the President and the Prime Minister discussed the ongoing negotiations to establish a normalized bilateral relationship between Iraq and the United States…In the area of security cooperation, the President and the Prime Minister agreed that improving conditions should allow for the agreements now under negotiation to include a general time horizon for meeting aspirational goals — such as the resumption of Iraqi security control in their cities and provinces and the further reduction of U.S. combat forces from Iraq. The President and Prime Minister agreed that the goals would be based on continued improving conditions on the ground and not an arbitrary date for withdrawal. The two leaders welcomed in this regard the return of the final surge brigade to the United States this month, and the ongoing transition from a primary combat role for U.S. forces to an overwatch role, which focuses on training and advising Iraqi forces, and conducting counter-terror operations in support of those forces.