If a time travel was possible it would always have been possible think before yo!
If a time travel was possible it would always have been possible think before you reply?
Time travel would only be possible from the point of its existance, so let's say it was invented right at this very moment, I would not be able to travel back a week in time, because a week ago, there was no time travel.
This is merely a variation on the GrandFather Paradox Theory, though slightly differently structured, fact is, you can only go forward in time from the intitial instance.
Think, you won't be declared the winner of the running event before you at the Olympic games by running backwards around the track, it's the same sort of thing as being implied here.