There’s a lot that’s possible, and there are plenty of movies that have taken advantage of the combination of a time machine and imaginative storytelling. Of course, there are also plenty of movies that skimp on the science in this regard.

No one remembers Timecop, Hot Tub Time Machine, or Bill & Ted’s Excellent Adventure for their incredibly accurate portrayals of time travel or a time machine. Idiocracy involves time travel only in the sense that time passes, even while inanimate objects (or people) remain inanimate. (Although, at least the “Time Masheen” is accurate.) Superman rewinds time to save Lois Lane’s life in the original Superman movie, but that’s due to superpowers, not science. Ditto for the recent Doctor Strange movie, or the cult classic Warlock, or Harry Potter and the Prisoner of Azkaban; using magic as the mechanism for time travel isn’t going to score you very many science points. In a great many films, time travel is more of a plot device than anything resembling scientific accuracy. Even Army of Darkness, fun though it is, doesn’t have a viable mechanism for the time travel it invokes.