Marvel's X-Men comics have long seemed obsessed with time travel - and there's a good reason for that. In 1981, Chris Claremont and John Byrne penned one of the most famous comic book stories of all time - "Days of Future Past." As iconic as this story may be, it's hard to believe it was only a two-part adventure.
Time travel has since become one of the most important - and, frankly, overused - tropes in X-Men comics. The X-Men have traveled to the past and the future, they've been joined by time travelers such as Cable and Bishop, and for several years the original five X-Men lived in the present day as something of a permanent fixture. Even the X-Men have begun to admit they're getting fed up with time travel - and no less a being than the Watcher has expressed frustration about it.
Time travel frankly seems like catnip for X-Men writers - and with good reason. Unlike most other superheroes, the X-Men aren't just do-gooders; they fight for a vision of mutant-human coexistence, in contrast with Magneto's dream of mutant supremacy or Apocalypse's intention to build a world based on the principle of survival of the fittest. In other words, the X-Men are about the future. Thus it makes sense for writers to be interested in exploring just what kind of world the X-men will build - or what the world would look like without them.
Of course, most of the futures glimpsed by the X-Men are dystopias. In narrative terms, this serves to create a sense of urgency about Xavier's Dream; the X-Men are the only slim chance the world has got of a peaceful future. Their opponents are the architects of these dark timelines. In the "Days of Future Past" timeline, the robotic Sentinels represent a future where man and mutant have been stripped of everything but hatred and prejudice. In "The Adventures of Cyclops and Phoenix," the titular heroes plunge into a timeline where Apocalypse rules the world. And in "Messiah CompleX," Bishop explores the question of just how far you should be willing to go to avert a dystopia; his willingness to kill a baby leads him to create an entire timeline where he had wiped out the entire human race.
All this is possible because of Marvel's general approach to temporal mechanics; they go by the Multiverse theory, that every action has the potential to create myriad branching timelines. This is the same approach Marvel is taking in the MCU as well, as seen in Avengers: Endgame, and it opens up the potential for a lot of cool time travel stories when the X-Men arrive on the scene.