"Bad Time: According to this, there's a single, correct timeline, one that needs to be enforced by some sort of temporal police. How can you tell which is the correct time? Well, of course, it's the one where someone formed a temporal police force."
Time Travel can cause a lot of damage. You could run into the Butterfly of Doom, leave behind a Timeline-Altering MacGuffin, try and bump off Hitler or create a Temporal Paradox. Therefore, in any society where Time Machines are easily accessible, deliberately altering time will be a crime. In addition, there may be people, most likely a police force or The Men in Black, who fight those who violate the sanctity of time.
The existence of Time Police is implicit in "time travel is illegal" scenarios - if they didn't exist in some form, it wouldn't be illegal. And if the illegality is anything more than a throw away line, people are going to worry about being found by the Time Police.
Of course, not all who police time are part of an official organization. In a setting where time travel is not easily accessible, there may be no laws to cover altering the timeline. Time Police in this case is any individual or group that takes it upon themselves to make sure that time plays out the way it's "supposed" to and keeps the timestream free of paradoxes. They may or may not be supernatural in origin, in this case.
Although Time Police are usually intended to be Lawful Neutral, with their only obligation being to keeping time clean, so to speak, they will often fight on the side of good anyway. Somebody ought to check the screening process. Although there's an increasing deconstructive trend to make them Knights Templar willing to do anything to preserve the timeline they want. Sometimes they might serve as a Weirdness Search and Rescue (although the latter characterisation might still be applied, with the one who helps the protagonists being the equivalent of a Cowboy Cop).
Oddly, if they're introduced in a long-running science fiction franchise, the question of where (when?) they were all the other dozens of times that time got messed up is barely handwaved if mentioned at all.
Also, Time Police covers other activities besides hunting down reckless or malevolent time-travelers. They spend most of their time protecting Adolf Hitler from being assassinated. (Lawful Neutral, remember?)
If there is no such group in a setting it may come down to the heroes to take care of the same. This can prove to be difficult. Time Police are very common in a world with Casual Time Travel.
Sister trope to Clock Roaches. While the Time Police are people, and therefore fallible and have agendas, who happen to use Time Travel technology to do what they see as their duty, Clock Roaches are animals, often non-sentient, who are inherently able to attack time travelers. In contrast to Time Police, they also happen to be Nigh Invulnerable, Eldritch Abominations, and/or Have Reserves. Either way, though, they both serve the same purpose as tropes: to provide an unambiguous in-universe reason why trying to change the past is a very bad idea.
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Anime and Manga
Doraemon: the Time Patrol often acts as The Cavalry in nearly every movie, since Doraemon and the gang have the unfortunate tendency to run into time-travelling villains.
Sailor Pluto from Sailor Moon is the guardian of the Time-Space Door and attempts to prevent and regulate time travel and people altering the time line. It's not explicit how she got this job, though Queen Serenity gave it to her at a young age. In her case, her powers over time extend to even freezing it around her, but she herself is not exempt from these rules. In the manga, she actually dies after using this ability, which she describes as a punishment for using it. In the anime, it's pretty strongly implied that she does die in the anime as well, and her later appearance is actually an earlier point of time from her perspective. Wibbly wobbly, timey-wimey.
In fanfiction, she tends to either abandon this role willy-nilly to ensure that the writer's time travel plot will work, or enforce it through extreme prejudice far beyond what she is ever portrayed as capable of. Particularly extreme fanfics portray her as a Machiavellian extremist that violently engineers the Crystal Tokyo timeline the series works on.
Flint The Time Detective: the main characters work for the Bureau of Time and Space, and were tasked with collecting all the Time Shifters who were scattered across time, despite being a couple of kids and an unfossilizedcaveman. This usually lead them to a famous historical setting where they would square off against renowned time thief Petrafina, who was collecting the time shifters for her own purposes.
Time Patrol Tai Otasukeman, the fourth installment in Tatsunoko Production's Time Bokan series. Both the two main characters and the villainous Terrible Trio work for the Time Patrol, whose job is to prevent alterations of history. Both the trio (which later becomes a Terrible Quartet) and the good guys have secret identities, the former as the Ojamaman, who try to alter history following the whims of a crazy guy in a green cloak who is nothing more than an AI created by their new fourth member, the real main bad guy, and the latter as the titular Otasukeman, who always manage to put everything back in place, so that we never see the Time Patrol actually do anything. Of course nobody ever discovers each other's secret identities until the very end.
Marvel Comics' Time Variance Authority, which ties up all the loose ends inherent in every single instance of time travel, and prosecutes the guilty parties.
Immortus, Lord of Limbo, considers himself a one-man time police force in Avengers Forever - his objective is controlling the Avengers in every possible timeline to prevent the human race from destruction by the Time Keepers. This extends as far as using the Forever Crystal to erase timelines which he feels are a lost cause.
Interestingly, the TVA hates the Avengers, as they're the single biggest perpetrators of time travel paradoxes. Seriously, a team with two Hank Pyms from different eras?
The Time Police in Paperinik New Adventures, who got around the many possible time-travel loopholes by building their HQ outside time itself: whoever is in the HQ isn't affected by any changes to the timeline, and can thus work to restore it.
The DCU has several protectors of the timesteam, most of them experienced time-traveling adventurers themselves: Rip "Time Master" Hunter, Waverider, the Linear Men, the second Chronos (the first and third were villains), the android Hourman, Booster Gold...
Hunter and Waverider were both members of the Linear Men at one, er, time. It's possible one of the many history-changing events they failed to do anything about has undone this.
The Time Masters are the new Time Police after the Linear Men were locked away. The Post-Crisis version of Rip Hunter founded and leads the group. Unlike the Linear Men, the Time Masters are more concerned with protecting the timeline from malicious time travellers.
One issue of Spirou and Fantasio ends up with the main characters being rescued by Time Police after they end up helping the first time traveler and become stuck in the past when his prototype breaks down. The Time Police state that he would need to be returned to his own time to continue his research, and imply that rescuing him is them performing their duty in a Stable Time Loop.
Wilbur Robinson claims to be part of a Time Continuum Task Force when first meeting Lewis in Meet the Robinsons. He's lying. The "badge" was a coupon for a tanning salon. The time machines are fairly new inventions and Wilbur is only 13.
Anderson doesn't shy away from giving them some Knight Templar tendencies either: In "The Only Game in Town", preserving the timeline that led to the creation of the Time Patrol means two patrolmen must alter the timeline and kill a Chinese expedition which, without the patrolmen's interference, would have brought word of the Americas back to Kublai Kahn.
Fredric Brown wrote three "The Short Happy Lives of Eustace Weaver" short stories about a man who invents a time machine and uses it to steal money from a bank with a time lock. In the first two, misunderstandings about the nature of time trip him up. In the third, Time Police arrive and execute him on the spot.
The Chronoguard in Jasper Fforde's Thursday Next, which, unfortunately for the protagonists, is terminally corrupt—barring a few honest holdouts—Thursday's son later reforms the force, only to discover that it's use of time travelling technology have the effect of irrevocably destroying humanity's collective cognitive abilities, and wipes the entire thing from the timeline.
When The Stainless Steel Rat suggests getting rid of a troublesome race of aliens by sending them forwards in time (to when the human race will be prepared for them) a member of the previously unknown Temporal Police materialises out of thin air and tells him it's forbidden.
This situation is analogous to the previous suggestion of sending the fleet to a parallel universe, blocked by the newly-introduced Moral Corps, whose authority supersedes even Inskipp, the director of the Special Corps. Their reasoning (perfectly valid) is that they have no right to dump the problem on humans in another universe. Unfortunately, the massive power requirements for transporting an alien armada to another reality limits the choice to only several "nearby" universes, all but one of which contain human life. In the remaining universe, humans have long ago subjugated the aliens but do not desire any more of them.
The Eternals in Isaac Asimov's The End of Eternity... do not, for the most part, act as this. Instead, they constantly tinker with the timeline to maximise the overall happiness of humanity. That said, the climax of the story is them trying to fulfil a specific aspect of history as recorded...
The Time Commandos in Simon Hawke's Time Wars series.
Agent of T.E.R.R.A. series of books by Larry Maddock.
Flying Saucer Gambit: Agent of Terra 1
Agent of T.E.R.R.A. #2: The Golden Goddess Gambit
Agent of T.E.R.R.A. #3: The Emerald Elephant Gambit
Agent Of T.E.R.R.A. #4. The Time Trap Gambit
The History Monks from Discworld may fit. If someone messes about with time too much they appear in order to prevent the worst effects. However it generally isn't so much a case of "avoiding changes to history" as "preventing time from shattering completely", which has happened "before".
The so-called 'Time Police' in Perry Rhodan's Magellan arc could be considered a mild subversion; rather than travel through time themselves, they just launched devastating attacks against any civilizations 'guilty' of using time machines in the present. Earth also once featured an (automated) alien installation that prevented time machines there from traveling back significantly beyond about 50,000 BC as part of a scheme to conceal said aliens' presence another one hundred and fifty thousand years earlier.
In J. K. Rowling's Harry Potter and the Prisoner of Azkaban, Hermione mentions in passing that she and Harry are breaking wizard law by meddling with time. The Ministry doesn't appear to have any way to actually enforce this, however, and they seem to assume that Time-Turners simply won't fall into the wrong hands.
The Ministry does keep them locked up in the Department of Mysteries. Rowling makes a point of having them all broken in book 5 so readers wouldn't assume time travel figures into the last two books.
In Christopher Stasheff's Warlock of Gramarye books, the inventor of the time machine sets up an informal time police organization (GRIPE) after his technology is stolen by people trying to change the timeline to defeat democracy.
The Eschaton in Singularity Sky and Iron Sunrise by Charles Stross is an AI using atemporal logic; it can violate causality by informing itself in the past of the results of future computation and observation. One of the things this lets it do is to observe others violating causality by the results in the future, giving the data to its past self, which can then prevent the incident occurring in the first place. It uses human agents as a first line of defense, and godlike overkill as the last to keep human history (and its own creation) intact.
The Time Purists in The Missing series basically "try to keep the timeline running correctly".
"Wikihistory", by Desmond Warzel, is told as a Web forum of the International Association of Time Travelers. Much mention of punishing rookies for killing Hitler occurs.
David Drake and Janet Morris had the ARC, that fought wars across different timelines.
The time cops of Time Scout are the BATF. Generally, they just keep people from profiteering from time travel and prevent looting of historical treasures.
A variant of your typical Time Police crops up in Roger Macbride Allen's The Depths of Time novel (and its sequels). Due to how faster-than-light travel works (ships fly slower than light to a wormhole, which sends them back in time to another wormhole), it's possible for ships to arrive at destinations before they departed, which would cause all sorts of problems with reality. The Time Patrol patrols the wormholes with battleships, destroying anything that attempts to get through without proper authorization. All ships have extremely paranoid computers installed in them, which will deactivate (or outright destroy) a ship if it thinks that it has wound up in the past. The Patrol can get news before it happens, which is locked away in vaults until the event actually happens.
In a short story prequel to the Morgaine Cycle, it's revealed that the precursors had Time Police whose job was to make sure that their Cool Gates weren't used to make large changes to the past. They did this not to preserve some "true timeline", but because if the past was changed too much it would lead to a truly catastrophic Time Crash. The Time Police eventually fail at their job, and the resulting Time Crash wipes out the precursors' galaxy-spanning civilization.
Ron Goulart's The Robot in the Closet and The Enormous Hourglass have a Time Travel Overseeing Commission. The Enormous Hourglass also has temporal Private Detective Sam Brimmer and his robot sidekick Tempo.
In Robert A. Heinlein's Future History series, after the invention of pan-universal time travel in The Number of the Beast, protagonist Lazarus Long and his allies grasp the horrific potential of a device that can transport anyone to anywhere, anywhen, in any reality, in the blink of an eye with zero power consumption. They form a Time Corps whose mandate it is to police the various timelines and fix any damage done by rogue time travelers, while at the same time identifying and recruiting likely agents from among those timelines. The plot of The Cat Who Walks Through Walls involves a running battle with exactly such a force; at stake is the "rescue" of an Artificial Intelligence capable of perfectly predicting the outcome of time manipulations.
Continuum: when it comes to enforcing the correct timeline, There Are No Police. There is, however, an intimidating cult of vigilante temporal enforcers known as Freelancers who act much more like a time Mafia than time police.
Voyagers! is an interesting concept, in that there's no actual evidence of meddling by anyone other than the Voyagers themselves. Their purpose is to make sure history went the way their records say it did. Is this a Stable Time Loop?
The Temporal Prime Directive in Star Trek, enforced by a variety of time agents who seem to have no relation to each other (and should be constantly getting in each other's way.) Captain Kirk harried the time agents to no end.
Voyager twice ran into Starfleet Time Police from the 29th century.
Then there was the whole Temporal Cold War thing in Enterprise, where it was implied that Daniels' side was mostly there to keep history going as it should have done.
Deep Space Nine had Temporal Investigations, which seems to be based in the present (theirs, that is, not the viewer's). The names of the two agents we saw were Dulmer and Lucsly.
In Doctor Who, the exact mission of the Time Lords was never made clear but it's implied that they somehow kept watch on time travel, dealing with any paradoxes and stopping people abusing it. And stopping the Clock Roaches from eating planets.
The Time Agency was initially thought to be humanity's Time Police. Between New Who and Torchwood, though, it's hinted to be a mix of this and opportunists.
Kamen Rider Den-O has the main protagonists acting as a form of Time Police, protecting the timestream from the Imagin, whose goal is to change the future by rampaging in the past. The Hero Ryotaro occasionally tries to bend the rules to help the Victim of the Week with whatever problem they've got. The fifth/seventh movie (Episode Yellow of the Chō Den-O Trilogy) introduces an actual Time Cop, who arrests anyone who alters history, good or bad; naturally, he ends up becoming the movie's antagonist as aside from anything else, by this point Den-O himself is a walking changed timestream.
The Guardians of Time in The Tomorrow People, presumably. The Guardians are a more advanced form of human than homo superior (called either homo novus or homo sapiens temporum), though it isn't exactly clear what their role is, as their appearances all involve them being lured into traps by villains seeking to exploit their ability to facilitate time travel.
Bernard's Watch: The Postman makes sure the watch's owner doesn't use it to commit crimes. He also ensures that time loops don't occur (such as when Bernard's cousin Lucy kept trying to rewind the watch).
Though no actual time police feature in Phil of the Future, Lloyd tells Phil that new laws regarding time travel were passed due to their family's intervention and was named after them, so there are stronger restrictions on Casual Time Travel. What that meant for Phil was that when his family returned to the future, they would be legally prevented from returning to the past.
All PCs in the time-travel RPG Continuum have this as one of their basic duties, although the Foxhorn Fraternity is the one specifically devoted to the task of rooting out Narcissists (time-criminals). Note that the RPG specifically mocks and derides the trope of the "Time Police" as a bureaucratic organization, and describes this as a misconception caused by our 20th-century prejudices. Keeping the timestream clean is everyone's responsibility in the Continuum.
In something of a subversion, said Guardians are incredibly overworked, paranoid and looking for help, and also fractured and corrupt — many have gone renegade. At least the current timeline seems relatively stable... for now.
The Infinity Patrol (and it's elite division ISWAT) from the default GURPS setting is partially tasked with policing alternate timelines. Unlike most time cops they are far from neutral as their main objective is to protect the interests of Homeline.
The Sidereals and other employees of the Bureau of Destiny in Exalted fills this role to some extent. Even if they exist in a universe where time travel is technically impossible, there is still a notoriously unstable Loom of Fate that has to be protected from disturbing elements (such as other Exalted or the creepythingslivingoutsideCreation).
In Chrononauts, the Time Repair Agency functions as this. A player who has 10 cards in their hand (one of the three ways to win the game) is made a new Agent in-game due to their skill at fixing paradoxes.
In Achron it turns out that the reason that the Grekim are determined to wipe out humanity is because humans were about to discover Time Travel and the Grekim don't want anyone 'muddying' the time stream.
What the laws are exactly is kind of vague in Where in Time is Carmen Sandiego?, but it's clear enough that it's illegal to steal historical artifacts from the past and the good characters are always concerned about history being disrupted.
City of Heroes introduced the Menders of Ouroboros in a major update, who try to recruit the players into helping them fix the timeline to avoid a mysterious future cataclysm only referred to as "The Coming Storm".
In The Journeyman Project, you are the Time Police. The world's first time machine, the Pegasus Device, is safely in the hands of the Temporal Security Annex, an organization devoted to protecting the timeline from those who would change history for their own gain (presumably, they use it for historical research); naturally, the player - Gage Blackwood, TSA Agent 5 - ends up having to save history from a xenophobic madman trying to sabotage Earth's entry into The Federation.
In the first game, the job of policing history is performed by allowing historical changes to happen, grabbing a backup disc containing unchanged history from 1 million BC, then cross-referencing with recorded history in the altered present. There are no safeguards if history is altered so that the TSA fails to be founded, but fortunately an improved time machine is used from the second game onward that doesn't necessitate returning to the present before the next jump.
The Bronze Dragonflight in Warcraft is pretty much this. In World of Warcraft, they enlist adventurers to help them battle the Infinite Dragonflight, implied to be Bronze Dragons corrupted by the Old Gods (who tried to manipulate time to free themselves in the novels), keeping the timeline intact.
Notably, that corruption is a recent thing. As of The Burning Crusade it was implied that the Infinite Flight were rebelling Bronze Dragons rather than corrupted ones. Since the major theme for Wrath of the Lich King has been "Old Gods corrupt things", the Old Gods are behind it. Likely this will change at least twice more before it's actually given a concrete answer.
Also notably, the leader of the Bronze Dragonflight and the leader of the Infinite Dragonflight appear to be the same person from different times. Time travel is confusing.
Furthermore, he has always known that he will eventually become the other leader and that he will then be killed by players allied with his past self..
As of the end of Cataclysm, Nozdormu has lost his immortal powers. While the Bronze Dragonflight is still doing their job as Time Police, the trading card game has introduced a faction called the Lorewalkers. They seem to be mortals that are ''also' stepping up to patrol history.
They are. They've appeared in game in the final patch of Mists of Pandaria examineing Timeless Isle, and island that doesn't flow properly with the rest of time.
Aeon from Castlevania Judgment turns out to be one of these, although this isn't revealed until you reach Death's story. Even then, you only learn the details and meet the perpetrator once you unlock True Story mode.
Phoenix from F-Zero comes to the 27th century from the 29th to stop a criminal from the future from mucking up time in the past. What this has to do with entering the F-Zero Grand Prix is never really looked into or explained; although he modified his machine to be on par with the "current" standards so as to not completely outclass the other racers, he makes it pretty obvious he's from the future.
The role of the Time Diver in Super Robot Wars's Alpha continuity is Time and Space Police; the job is initially taken up by Ingram Plisken who is subsequently killed and tries to take over the body ofAyin Barshem, later known as Cobray Gordon. The latter's force of will allows him to resist the possession, and eventually the original Time Diver passes on his title and responsibilities before moving on.
This is the purpose of the Sentinels of Hallifax in Lusternia. They spend most of their time cleaning up the messes of their fellow Hallifax guild the Institute.
The Sequel Police from Space Quest IV are a villanous example, trying to hunt down and kill a time-traveling Roger on Vohaul's orders.
Magical Girl Lyrical Nanoha A's Portable: The Gears of Destiny has the Florian Sisters, who were tasked by their father to serve as the Guardians of Time, who will protect the Destined Fate. The plot begins when Amitie Florian goes after her sister Kyrie, who had gone rogue in a desperate attempt to find something in the past that could help her father achieve some progress in his planet restoration project before he dies.
In the Ambitions expansion for The Sims 3, Sims using the Time Machine occasionally have narrow escapes from a mysterious "Keeper of Time." Everything only appears in flavor text, though, so there are no in-game implications.
Deep Time from Starslip Crisis serve both as a parody as well as a Deconstruction. Originally, they appear fairly uninvolved with the plot, only hunting rogue time travelers, but eventually they get into a jurisdictional dispute with the present over a time machine. This leads to a war in which the Future battles the Past, and Deep Time can't do anything without erasing their own existence, while mankind's present's government tries to beat them by banning time travel research, but continue it in secret anyway, leading to Deep Time's existence. Eventually, they erase the entire timeline and start over to ensure their own existence comes to pass, though the past would have won for want of a spork.
Superjail! has Time Police who live in what looks like an 80's vector-graphics arcade game, and whose dialog is all sung Motown-style for no apparent reason. They pop in out of nowhere and arrest the Warden for the crime of conquering the Earth on a whim — something he hasn't actually done yet. Like most Superjail! stories, this ends in a bloody massacre breaking out, with a time loop thrown in for good measure.
Futurama had the Vice Presidential Action Rangers int the What If? episode, a "group of top nerds" tasked, by the U.S. constitution, to protect the space time continuum from disruptions. It consisted of Al Gore, Stephen Hawking, Nichelle Nicols, Gary Gygax, and Deep Blue. Fry lampshades the odd assortment.
Fry: I thought your job was to cast the tie-breaking vote in the Senate. Al Gore: And protecting the time-space continuum. Read your constitution.
On Time Warp Trio time travelers are responsible for making sure other time travelers like the series Big Bad don't screw up history. It's mostly informal, but they have Time Agents posted at certain at risk times/places, and they do travel back specifically to stop him at least a few times.