It makes no sense to tell the enemy when you plan to start withdrawing. All the terrorists would have to do is mark their calendars and gather their strength — and begin plotting how to overthrow the government and take control of the country of Iraq. I believe setting a deadline for withdrawal would demoralize the Iraqi people, would encourage killers across the broader Middle East, and send a signal that America will not keep its commitments. Setting a deadline for withdrawal is setting a date for failure — and that would be irresponsible.
President Bush Rejects Artificial Deadline, Vetoes Iraq War Supplemental (May 1, 2007)
President Bush Rejects Artificial Deadline, Vetoes Iraq War Supplemental[georgewbush-whitehouse.archives.gov]