If the abduction to Tralfamadore is a psychosis / hallucination...
I've heard it theorized that Billy's abduction to the planet Tralfamadore is all in his mind. It is a hallucination or psychotic episode that has both mental (his war experiences) and physical (his head injury after the plane crash) origins. This provides a logical -- or "real world" -- explanation for a bewildering, other-worldly phenomenon that just isn't part of common experience.
But if that is the case, how does one explain the time travel? Billy had his first experience with time travel in December 1944 when he was hiding from the Germans in the forest (and here I'm going by the novel; I don't know if the movie differs). (1) We can't dismiss the time travel episodes as merely an escape fantasy because he saw FUTURE events of his life which actually came true 10 to 20 years later. (2) It's not an effect of post-traumatic stress disorder because he's right in the middle of the trauma -- not remembering it afterward -- when the time travel takes place. (3) And it can't be the result of his head injury because that didn't happen until the plane crash in 1967. (EDIT: My mistake. The plane crash was in 1968; the abduction to Tralfamadore was in 1967.)
What logical explanation is there for the time travel? Is it simply that Billy was clairvoyant?