Mother Jones has a timeline of Bush's military service record. Key events:
May 26, 1972:
Transfers to Alabama Guard unit so he can work on Senator William Blount's reelection campaign. According to his commanding officer, Bush never shows up for duty while in Alabama, nor can anyone confirm he ever serves in the Guard again. [Emphasis added]
August 1972: Bush is grounded for missing a mandatory physical.
November 1972: Bush returns to Houston, but never reports for Guard duty. [Emphasis added]
(via Testify!, which says, "[T]his is something that Bush should clear up for us." At least!)
Update: Here's more from ABC News back in 2000: "Gov. George W. Bush’s campaign workers have concluded that no documents exist showing he reported for duty with the Alabama National Guard as ordered in 1972." And this from the Boston Globe:
In his autobiography, ''A Charge to Keep,'' Bush said he flew with his unit for ''several years'' after finishing flight training in June 1970. His campaign biography states that he flew with the unit until he won release from the service in September 1973, nine months early, for graduate school.
Neither assertion is true. Bush flew with the 111th for 22 months, until April 1972, and never flew again. Bartlett said last week that he could say unequivocally that Bush was not grounded by his superiors. Asked that question last July, Bartlett, after conferring with Bush, was more equivocal: He said Bush could not recall ever being grounded.
...The official record that chronologically lists Bush's service includes no evidence of service between May 1972 and October 1, 1973, the official date of his discharge. [Emphasis added]