That's just what the House and Senate supplemental appropriations bills require of Bush if he wants to continue the war in Iraq.
What's in the House and Senate legislation:
July 1, 2007: Bush must certify Iraq is making significant progress in securing the country, sharing oil revenues equitably among all Iraqis and other political and military benchmarks. If Bush does not provide the certification, an immediate withdrawal of U.S. troops must begin and be completed by December 2007.
October 1, 2007: Bush must certify Iraq achieved the benchmarks. Otherwise, U.S. troops must begin withdrawing immediately and be out of Iraq by March 2008.
September 1, 2008: If both of the above certifications were provided by Bush, U.S. forces would begin withdrawing by March 1, 2008 and would finish by September 1, 2008.
March 31, 2008: U.S. troops would begin to withdraw from Iraq no later than four months after enactment of legislation "with the goal" of finishing the withdrawal of combat troops by March 31, 2008.
Both bills would allow some American troops to remain in Iraq to help train Iraqi forces, to protect U.S. personnel and to conduct counterterrorism operations. But combat troops would be withdrawn.