The Atavachron was a temporal device utilized on the planet Sarpeidon to travel back through time. The atavachron altered a time traveler's cellular structure, making it possible to survive in earlier environments, but also making it impossible to return to the present without reversing the cellular alteration.
In 2269, Mr. Atoz used the atavachron to send the residents of Sarpeidon back in time to escape the danger of their sun going nova. A travelor's destination was chosen by the person from the planet's own history, which were programmed on circular disks.
Kirk, Spock and McCoy were accidently transported back into the planet's history. They were able to return to the present, since they were not treated for cellular alteration prior to the use of the atavachron. (TOS: "All Our Yesterdays")