I am not questioning this.. It is possible to go to a place where time passes slower, and then come back to find ourselves in 2500...
This could be achieved while travelling at the speed of light..
Earth's one year might be equal to one hour during that travel.. and when such body will come back to earth, many more years might have passed already..
It is also possible that we go near a black hole, where time passes quickly, and we pass 50 years, but on earth only one year has passed.. when we come back, we are probably 70-80 years old, only to find our friends just one year added in their age.. but still, time has passed on Earth.. not as much as near a black hole, but still passed..
What I am saying is that there is no way to reverse the time.. at least not with the current theories.. and that's the reason why we haven't found anyone from future till now..