Yeah, I don’t think it’s time travel. I think the “unstuck in time” is a metaphor for his PTSD flashes.

“Bid Time Return” (a.k.a. Somewhere in Time). By Richard Matheson*
The book is good, and actually the 1980 Movie adaptation starring Christopher Reeve, is excellent.

Most unique time travel mechanic I’ve ever seen. It’s so good I don’t even want to spoil it here if you don’t know. It’s a great read (or watch if you don’t have time).

*Yes, that Richard Matheson, who also did ‘I Am Legend’, ‘Duel’ (which started Spielberg’s career), ‘The Shrinking Man’, ‘Button Button’ (aka The Box), ‘What Dreams May Come’, and many more.