So you want to know about THAT, eh?
They aren't time machines, just really fast space vehicles that use supergravity waves to stretch space, allowing faster than light travel. These vehicles ARE capable of time travel, but people generally prefer NOT to do it because they stand a chance of getting lost in time, which brings me to what time travel REALLY is.
Time travel does NOT alter the past, because everytime you travel to the "past", it is not YOUR past that you travel to. It is an alternate reality among infinite realities, where things may be a little bit different than the past you knew. If you start to alter things in that reality, you will NOT "kill your own grandmother" because she lived in another reality to give birth to you. For example, in the past that you knew, the Eiffel Tower was painted black. However, if you travel to the past, in that past the Eiffel Tower may be painted red or another colour. Thus, time travel is actually "reality hopping" through infinite universes that are slightly different from each other. When you time travel, you may not be able to find your way back to the same universe you lived in, which makes time travel a rather frivolous and unecessary thing to do, unless you want to create a time loop in your universe, which is another stroy.