That's due to a relativistic effect called time dilation. And experiment and observation have repeatedly validated this effect; so it's real. When you move, time for you relative to your sister watching you walk slows down just a smidgen. It's not noticeable because it's so small.
But a group of physicists traveled commercial airlines at about 1/6 mi/sec (600 mph) around the Earth and when they landed, the atomic clock they took with them was just a tad slower than the one they left behind at the terminal. And that difference was how far into the future they went.
So there you are. Travel into the past...no. Travel into the future...yes. And time is just the unfolding, the happening, of events.
One more point I almost forgot...when time slows down, like for those physicists, the rate of unfolding events slows down. For example, their heartbeat rates would, on average, slow down just a bit. Heartbeats, you see, are events and their rate of unfolding slows when time slows.