Time Travel Logic (#SFWApro)

Time travel is, as far as we know, impossible. So it’s not surprising that time-travel films, as they involve impossible things happening, frequently ignore logic.
That’s not to say that even films that are logical are logical by real world standards. In Before You Say I Do, for instance, the protagonist goes back in time by wishing as he passes through a yellow light. The story finishes when he unthinkingly wishes as he passes through a yellow light and returns home. Not real-world logic or even explained (God? Fairy godmother?) but it does have a dramatic symmetry. And that usually works.
By contrast, we have Black Knight, in which Martin Lawrence goes back in time when he grabs a magical medallion out of the water. When he returns home after his adventure, everyone insists he hallucinated while almost drowning. Then he falls in the water again, and finds himself in ancient Rome. Except there was no medallion to grab, so doesn’t that imply the first trip really was just a hallucination? Which means there wasn’t much point to it. No dramatic symmetry.
Another form of illogic—a very common one—shows up in Pokemon: Arceus and the Jewel of Light, which I caught this weekend. As the super-Pokemon Arceus attacks a small town, Ash and his friends are hurled back in time, giving them a chance to change the events that turned Arceus into an enemy of humanity. They succeed, return to the present, and see Arceus suddenly pause mid-attack as the altered memory of the past catches up with him. They’re friends again!
Of course, if history was changed, there never should have been an attack. But this is a common movie illogic, presumably because it builds drama. In Trancers, for instance, the villain goes back to the 20th century to eliminate the ancestors of Los Angeles’ future leaders. When an ancestor dies, the leader “simultaneously” ceases to exist, rather than never existing at all.
Some films attempt to deal with this. In Time Shifters, the time-travelers’ based in the future is insulated so that changes to the time-stream don’t affect it immediately. In Steyn’s Gate, an anime series I’m working through, the protagonist Okarin discovers that he alone of his friends retains any memory of the previous timeline once they change it. They’re the exception though.
And of course, there’s the grandfather paradox — the idea that if you kill your grandfather, you’ll never exist to go back and kill him. In Against Time, for instance, has Robert Loggia return from the future to change his tragic past; having succeeded, he has a happier life and never has to go back and fix his past. So how did it happen?
One solution, of course, is that the time traveler created an alternate timeline. Unless the film actually states that though, I’m not willing to assume it, partly because it’s dramatically unsatisfactory. If the original timeline where Loggia’s a tormented drunk continues to exist, the film loses a lot of its punch. So if the creators say history changed, I’ll accept it despite the illogic.