In "Back to the Future Part II," Marty McFly travels to October 21, 2015, to save his children, yet to be born in "Back to the Future's" 1985.
http://edition.cnn.com/2015/10/20/entertainment/back-to-the-future-day-feat/
http://edition.cnn.com/2015/10/20/entertainment/back-to-the-future-day-feat/