While in Star Trek we do not see any time machines so named, we do see time travel. Repeated time travel to the point of banality. Different authors handle the issue in different ways. Each will be asked to speak for themselves.
Star Trek: Athena Edit
Time travel in Star Trek: Athenagenerally has involved the Slingshot Effectmanoeuvre about stars, instead of relying on machines. The Guardian of Forever was a focal point in the episode The Ripples of Timewhen the crew has to go back into the best to reset the time line after an officer on a ship that the Athena was escorting went through the Guardian and altered the timeline to prevent World War III from occuring.
A time machine is the focal point of the upcoming novel "Dawn."
As a general rule, a time machine can only take people to time periods in which the time machine is continuously active, and this condition is adhered to in "Athena" stories. Time travel has generally been used to create alternate history stories to examine what could happen if history has changed, such as "The Ripples of Time" and "Tribal Rights," and to allow members of the crew to experience life in a pervious time period, "One Valiant Effort." This approach has been used rather than to recreate the time periods on a holodeck or to have the ship visit an alien world that has a similar historical situation compared to the one being investigated, since these would not have the same impact as actually seeing these changes in action, or else it would strain credibility to find an alien race that exactly duplicates some time period on Earth. "The Big Bright Boom" shows another use for time travel, investigating scientific phenomena, supernovas in this example, that are simply not available in the time frame of the story.
Banshee Squadron Edit
Time travel exists in the Banshee Squadron continuity, and one time travel story has been written to date. ("Humanity") Federation timeships from the 29th century patrol the multiverse just as they do in canon, and the Temporal Cold War was/will be fought, again just as in canon.
Wormholes and other gateways, including the Black Gate, the Guardian of Forever, the Bajoran Wormhole, the Barzan Wormhole, Vaadwaur underspace, and Iconian gateways are all part of or at least tap into a universe-spanning network that is woven into the natural fabric of spacetime, yet shows some indication of being an artificial construct.
Star Trek Dark Horizon Edit
Epiphany Trek Edit
Out of GameEdit
Epiphany Trek avoids time travel plots. The fact of time travel is not disputed or denied, it is quietly ignored hopefully to death. I am of the opinion that time travel has been over used and badly over used, often with the effect of The Reset Switch to make it all go away. As a result I leave the tired worn out plot device alone.
Time travel is a fact of physics. The Guardian of Forever is the most dangerous device in existence because it is a time travel device locked into the very existence of the universe. You don't dare use it, and you don't dare get rid of it. The Epiphany Trek universe employes the Laws of the Conservation of Time
Starfleet ships have time traveled, it is possible to use a time warp. It is strongly, strongly advised that you do not. The Captain that messes with time travel will have a lot of questions to answer at the board of review.
It is understood that the nature of the universe is highly mutable. Alternate universes abound. Because of this here is no such thing as a core or "pure" time line. All are equally valid. The problem with time travel is you do not move solely up and down the time stream, it does not work that way. Time travel is alternate universe travel. Indeed the minute you insert yourself into a past time line you create an alternate universe by your very presence. The mutability of the universe and the nature of alternate universes are the only reason time travel does work. You never get back to the universe you left. There will be differences either minor or even significant. Those that build dedicated time machines learn this to their grief. You cannot go home again. Dwelling too long on the total mutability of the seemingly immutable universe has been known to drive people insane. The study of Hyper-dimensional physics requires the applicant pass a battery of tests for mental stability. That way lies madness.
Light Trek Edit
Star Trek Outwardly Mobile Edit
Out of Game - I dislike time travel as a plot device in Star Trek. I'd prefer to avoid it if possible. This doesn't mean I won't use it if there's a good story involved, but I'd rather find a different way to go. In my humble opinion there are some tricks to a star trek story - and time travel is often used as a cheap and easy way to get those elements into play.
However, That being said, I haven't seen a Trekcreative player use time like that in a story, yet.
In Game- The Universe hates paradoxes. In terms described by GURPS Time Travel - the Universe has a very limited elasticity. Probabilities are real - and can change. The Universe responds to time travel with altered probabilities. Time machines draw bad luck and destructive circumstances.
The math works best - and therefore reflects a happier universe, metaphorically - when the time machine just melts down and/or explodes (The potential energy of the change in time and space is expressed as heat.)
Second best is when a time machine creates self- referential causality loop of it's own destruction. A series of events that results on the destruction of the machine, the death, injury or emotional misery of the machine inventor, makers or users, but is functionally neutral from outside the context of the attemmpt to build and use the machine.
Third best is when something possible, and probable happens to the time machine and it's associated users, to prevent or obviate the use.
Fourth best is when something posssible but less likely happens to neutralize the machine and it's associated users.
The more precautions the time machine makers and users take to avoid likely problems, the more unlikely and strange the problems that beset the time machine. After a certain point, even the most oblivious and committed methodological naturalist will begin to percieve that there is a force at work drawing disasters and problems towards the time machine.
In ST-OM you will not be able to take a time machine back in time and kill your grandfather. You will get hit by a bus trying to do so. The universe will protect itself from paradox in this fashion every time and with increasing viciousness. Time travel is not worth it.