Quizmonster - yes, indeed; that is the whole point which Steve was making. It's normally known as the "grandfather paradox".
But yes, actually, it is possible to travel in time. The passage of time slows down for travellers according to their speed - the faster, the more severe. If you travelled for a long time at near the speed of light, you would be able to go away for a few months and come back to Earth and find that thousands of years have passed. This has been confirmed by comparison of highly-accurate atomic clocks on aeroplanes being compared with those on the ground. The person who has travelled most in time is a Russian cosmonaut who has spent more than a year or two in space, orbiting around the Earth and several thousands of miles per hour. The cumulative effect of all his travels is that he has gone approximately one-fiftieth of a second into the future, i.e. his age-wrinkles are appearing one-fiftieth of a second later than they would have done without his travels.