Temporal Prime Directive

The Temporal Prime Directive is a fundamental Star Fleet principle.

This directive states:
All Star Fleet personnel are strictly forbidden from directly interfering with historical events, and are required to maintain the timeline, and prevent history from being altered.

• The directive also restricts people from telling too much about the future, so as not to cause paradoxes or alter the timeline.

In-Universe History

When Captain James T. Kirk, Spock, and Dr. Leonard McCoy were transported from 2267 to 1930, Kirk acted to preserve the timeline by allowing a civilian to be killed in a traffic accident, even though he had become friends with her. Although the Temporal Prime Directive was not mentioned by name, his actions preserved the timeline which culminated in the birth of the Federation and Star Fleet.

It is unclear exactly when the Temporal Prime Directive came into effect. In universe, the first mention of a Temporal Prime Directive was by Captain Jean-Luc Picard in 2368.

Picard postulated that the existence of such a directive might be what was preventing a traveler from the future from aiding him in averting the destruction of a planet.
It was clear that Picard was unaware of any formal Star Fleet equivalent to a Temporal Prime Directive at this date.

Captain Kathryn Janeway seemed to be aware of the Directive's existence in 2371, long before the temporal incursion by Captain Braxton in his timeship in 2373, which implies that the Temporal Prime Directive was not only a 29th century regulation.

On Deep Space 9, Julian Bashir referred to the similar "temporal displacement policy," something taught at Star Fleet Academy, in 2024 when he traveled back in time to that year from 2371.
"Star Fleet Regulation 157, Section 3, Paragraph 18" also seemed to relate directly to time travel. Of course, determining exactly "when" a directive relating to time travel came about can be difficult.

The events of the Temporal Cold War may suggest that the regulation has been rescinded, at least temporarily, by the 31st century. However, the Federation faction in the Temporal Cold War is dedicated primarily to keeping the timeline intact and preventing the other factions from interfering with it, which would be completely in keeping with the Temporal Prime Directive. On a few occasions, a temporal agent had to tell the crew of the Enterprise NX-01 some things about the future in order to preserve the timeline.