The time-gate was an almost forgotten invention of Théo Guderian in the early part of humanity's acquaintance with the rest of the Galactic Milieu. Due to peculiarities in temporal mechanics, it could only function in a specific geographic location in France and only send people and things back in time approximately 6 million years (called the Pliocene epoch) to the same location at that time. After knowledge of its existence was more widespread within the Milieu, it became a gathering point for a variety of people that would be known as Exiles.
Return time-gate Edit
Technically there were eventually two time-gates. At some point, smuggled technology, brought to the Pliocene by escaping Rebels, was used by their children in cooperation with the Tanu (at the dictates of the presiding human king, Aiken Drum) to construct a working return gate to the "present". How this was used beyond allowing some Pliocene residents to go to the Galactic Milieu is not clear.