It depends on the nature of the time machine (i.e. how it proposes to travel through time). But essentially all theoretical attempts at a time machine are practically impossible (i.e. creating massive amounts of space time curvature).

The short answer is, no one knows for sure.If it's possible, then it opens up huge paradoxes. For example, what would happen if you traveled back in time and accidentally killed your grandfather before you were born? If you weren't born, who killed him? And so on.

Time travel is probably impossible in a practical sense. If it isn't impossible, there would be no reason why you couldn't go back that far.