Quote from : X-Men: Days of Future Past Movie

Kitty Pryde: [prepping Logan] Basically, your body will go to sleep while your mind travels back in time. As long as you're back there, past and present will continue to coexist. But once you wake up, whatever you've done will take hold and become history. And for the rest of us it'll be the only history that we know. It'll be like the last fifty years never happened, in this world, in this war. The only person who will remember it is you. All right, Logan, I need to you clear head and to stay as calm as possible.