Assume I can warp time. So, I can create a portal to eighty-five million years ago (like they did in Terra Nova). I walk through the portal expecting to see dinosaurs, however bam - I see an open space. It would be stupid to assume that our Earth is the absolute center of space - it moves in this space constantly. So, exactly eighty five million years ago (due to that extra half a second each year) our Earth could be on the opposite side of the Sun. And yeah, hello open space and please wait for half a year till the Earth flies to you. Wait, no - the Sun itself could be million miles away. To create a portal that would constantly teleport people to 85 million years ago _to the surface of Earth_ the portal's destination will have to repeat the movement of the Earth _exactly_. And even if calculating the Earth's trajectory is possible - good luck manipulating the other end of the portal.

We are never going to do time travels.