For Jack, the timer is symbolic of his conflict with George Hatfield, the fight which loses him his job and pushes him toward the Overlook. George, a student on Jack's debate team, stutters and therefore is never fast enough in debate. George accuses Jack of setting back the clock timer to rob George of part of his five minutes. Though he's sober during his conflict with George, he still completely loses control and becomes violent. The incident reminds Jack that the violence was inside him in the time before the drinking, and is therefore separate from it.
The timer also points to the haziness and anxiety surrounding time throughout the novel. In Jack's walking-dream, the timer is attached to a string of dynamite, foreshadowing and counting down the explosion of the boiler, and the end of Jack and the Overlook.