In a classical example of time, time is and was the exact center of mass of the universe. prior to the universe as we know it, the universe was a point of mass infinitesimally small. The exact center of mass was a point called time. When the universe exploded, based upon external forces as some would propose ie the hand of God, the point called time was put in motion, always maintaing itself as the center of mass of what was the original universe plus the additional force introduced by God.
The line transcribed by associating all the points of the center of mass has been defined as the time line. This line is not linear, quite the contrary, it twists and turns. If you could approach any point on the line, you would experience the universe as it existed at that point in time.
Time travel can only be accomodated by travelling back in time for it is there that the time line exists, not in the future. There are additional limitations and anomolies which must be considered. Let me know if you are interested and I will provide you with further information to help your understanding of this phenomenom.
Somehow, that doesn't sound right. Assuming there is a timeline, an entity living somewhere on that timeline should be able to travel forward to our time as well as backwards, as you say that we can. Just as they dwell somewhere along the timeline, then we also dwell somewhere along it and not necessarily at the end of it. The timeline would extend ahead of us as well as behind us. No?