Actually, time travel to the future is in fact possible in the sense of traveling to someone else's future. Astronauts who go into space and return have in fact travelled into our future. The time that elapsed for them is less than the time that elapsed for those of us back on earth. The difference, in practical terms, is insignificant. But it is a fact and it is measureable. If those astronauts were able to travel through space at speeds approaching that of the speed of light, when they returned to earth, a much greater time would have elapsed on earth, the amount of elapsed time depending upon at what speeds and how long they had travelled.
There is no known way of similarly traveling back in time however.
Well, no. The fact that they may have aged more slowly than you or I does not mean they have time-travelled; it simply means they have aged more slowly. This is considerably different than travelling into Earth's 30th century, or Earth's 12 century.