Apart from the many versions of A Christmas Carol, the Christmas movie most recognizable for temporal tinkering is It's a Wonderful Life, Jimmy Stewart's box office flop that has been revered as one of the best films of all time.  In it, George Bailey wishes he had never been born, and thanks to the intervention of the angel Clarence he gets to see what his wish would have meant.

The film begins by telling snippets from the life of George Bailey, as he saves his brother from drowning, prevents the local pharmacist Mr. Gower from making a deadly error in filling a prescription, meets and marries the wonderful Mary, saves the savings and loan association so the evil banker won't destroy the town, and in general makes sacrifices himself for the benefit of others.  It leads to a moment when that evil banker manages surreptitiously to steal money needed to keep the savings and loan solvent, and George despairs of life.  He would have committed suicide had the angel not intervened, throwing himself in the river so George would rescue him.  Then when George is blaming himself for the present crisis, Clarence changes the past such that George Bailey never lived.

It might be argued that this is not a time travel film.  After all, no one travels through time.  George Bailey does not watch events unfold without him.  Rather, he finds himself abruptly in a world in which he never existed, in a world in which the things he did were never done because he was not there to do them.  History has been changed; it was changed based on George's wish, although George did not travel to the past to change it.  Clarence changed the past; he did not travel to the past to do it, but he did reach back into the past and cause the change, based on information in the present.

It thus might be argued that this is a grandfather paradox.  George has not exactly killed his own grandfather, but he has undone his own existence, and now he does not exist to make the wish that does this.  We ought to be trapped in an infinity loop, since undoing George's existence undoes the reason for undoing it, and so undoes the undoing.  In fact, if George was never born, he should not be able to know what the world would have been like without him.

It might be resolved by adopting a parallel dimension explantion:  George has moved sideways into a universe in which he never existed, and then returns to his own universe after an hour or so of studying that world.  It means that the bleak dreadful world without George Bailey also exists, though, which is not the message the movie intends to send.

It seems the best explanation is a divine one.  God knows all the worlds that might have been, and could show George one of them without destroying what is or requiring that what never was must be.  Clarence is, after all, an angel, God's messenger to help George, so tapping such divine perspective is not entirely unreasonable.

There are a few points in the story that might be challenged.  Most of them concern the notion that the absence of George Bailey would not have been filled by someone else.  Mr. Gower probably would have hired a different assistant, and that assistant might have recognized the mistake as easily as George.  Granted that Harry drowned, the air force would have had some other pilot in his place, who might have been that medal of honor winner who rescued the men Harry was not there to save.  Someone else might have saved the savings and loan.  Mary might have married someone else.  Still, it is not certain that anyone would have stepped into these roles, and the alternate history is at least credible.

The film works under a version of replacement theory that allows for the divine intervention suggested by the plot, and also under that version of divergent dimension theory in which every possible universe exists and George can travel sideways to the present moment of another.  It is also a solid inspirational story that should make most of us feel as if we may have done more good in the world than we can see at this moment.