Can only travel fowards in time, the past doesn't exist. Only conceivable way would be travelling to a paralell universe almost identical to our's, but in a past time period. Time's passage is effected by velocity as per relativistic time dilation when moving near the speed of light. They theorize if you travelled precisely at lightspeed time would stop completely, but that's impossible because as velocity increases so does mass and thus the energy required which at lightspeed would be equal to more energy than is available in the entire universe. But they've worked out the Russian cosmonaut who was on Mir for about a year, due to that velocity and duration is like a second or two younger than a groundbased counterpart. So the time travel effect is confirmed science. Go fast enough, time slows down for you the traveller while proceeds at the normal Earth-bound rate for everybody else. So a time machine would simply be some spacecraft, and if it were solely for travelling into the future would simply orbit something while it does its thing.
If it's you have to stick your two cents in, but it's only a penny for you thoughts, what happens to the other penny?
"I'd rather be hated for who I am, than loved for who I pretended to be." - Me.