Temporal displacement policy
From Memory Alpha, the free Star Trek reference.
The Temporal Displacement Policy is a series of guidelines which, like the Temporal Prime Directive, instruct Starfleet officers how to act so as not to change the timeline if they somehow travel out of their own time.
Dr. Julian Bashir referred to the policy in 2024, having traveled back from 2371 with Commander Sisko and Jadzia Dax. (DS9: "Past Tense, Part I")
[edit] Apocrypha
In the non-canon novel Federation, the policy also specified that if a starship from the past were to encounter a starship from the future they should maintain communications silence unless the future starship hailed them and reduce the viewscreen's resolution to its lowest point. This particular note was contradicted in TNG: "Cause and Effect". It also stated that if the ship from the past was facing a situation in which it was not likely that it could be saved, then the starship from the future could take the crew of the past starship on board. This is consistent with the reaction of the alternate USS Enterprise-D crew in TNG: "Yesterday's Enterprise", as the temporally displaced Enterprise-C was initially invited to rejoin Starfleet in the 24th Century rather than returning back through time to their near-certain demise. It was not until additional information came to light that the extraordinary decision was made to send them back anyway, despite the overwhelming likelihood of destruction.
