Released in the United States on May 23, 2014, Days of Future Past is simultaneously a prequel and sequel to the X-Men film franchise. Several cast members from the original X-Men trilogy appear in the future segments of the film.
"The ultimate X-Men ensemble fights a war for the survival of the species across two time periods in X-MEN: DAYS OF FUTURE PAST. The beloved characters from the original “X-Men” film trilogy join forces with their younger selves from “X-Men: First Class,” in an epic battle that must change the past – to save our future."
Professor X and Magneto used time travel so their allies can defeat Bolivar Trask. By doing so, it will prevent the cure on mutants.