According to the St. Petersburg Times, there was an "official schedule" that had Bush leaving his hotel at 8:30am.

According to the Washington Post...

"Bush's motorcade left for the school at 8:30 a.m. As it was arriving, pagers and cell phones alerted White House aides that a plane had hit the North Tower of the World Trade Center. Bush remembers senior adviser Karl Rove bringing him the news, saying it appeared to be an accident involving a small, twin-engine plane."

According to the BBC, they are also going with 8:30.

"08:30:00
Sarasota, Florida

The Presidential motorcade heads for a local school. The morning intelligence briefing over, the threat level considered low. Before the day ends George W. Bush will have embarked on a remarkable odyssey - unlike any previous President."

According to the Washington Times, their account of him leaving is at 8:39...

"So after posing for pictures with resort maintenance man Kenneth Kufahl and local VIPs, he climbed into a Cadillac limousine and set out at 8:39 a.m. on the nine-mile trip to Emma E. Booker Elementary in Sarasota."

Who's willing to bet that the Post, and the BBC were going by the "official schedule", Bill Sammon of the Washington Times, was told 8:39 from "extensive interviews [...] in the Oval Office and aboard Air Force One" but he really left at 8:46?