That point is often brought up. But we don't really know how a time-machine could potentially work, so why is teleportation a necessity? If you want to travel to the future you would abuse time delation. While all means to do so pose certain problems none of them involve teleportation. If traveling to the past would work in a similar way the time-machine would move trough every moment until it reaches its destination. If it never has to leave earth or its orbit to do so it would always be affected by gravity and as such end up where you want it to.