[tessarji]

Well,

Scientists who have 'constructed' relativistic time machines (in a thought experiment sort of way) have indicated that they cannot be used to travel before the construction of the machine. This is basically because they are superimposing points in space-time from two different eras, therefore they do not have access to points that do not overlap with the time machine's own existence.

If you are making a machine that operates outside conventional relativistic theory, then of course anything could be possible depending on the structure of your physics.