Let's say you can make a wormhole.
Put one end on a space ship and send it off at the speed of light. When it returns 20 years later, that end of the wormhole will be almost 20 years younger.
You could then travel from your end of the wormhole through to the space ship's end and arrive 20 years in the past!
Or if you went the other way, you'd be 20 years in the future.
Just remember to watch out for Daleks!