Time Police

""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.""

- Lore Sjöberg, on various forms of time travel, Alt Text

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 True 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.

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.

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.

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, she fakes her own death at a different part of the story, though she never does explain why she had to do it and why it was okay to reveal herself again later in the story.
 * 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 unfossilized caveman. 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 instalment in Tatsunoko Productions'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, 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.
 * Vector Prime has this role in Transformers Cybertron and, by implication, the entirety if the Transformers Multiverse.

Comic Books

 * 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 . 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.
 * The Time Police in Jughead's Time Police. They were equipped with cool future technology.
 * The Temps Aeternalis in The Umbrella Academy. It seems like their main function is to carry out assassinations, or as they call them, "corrections".
 * Their purpose is to maintain the status quo.
 * Valerian is about the adventures across time and space of a member of the Time Police.
 * This short comic from ~2000 AD~, written by Alan Moore and illustrated by Dave Gibbons (before they collaborated on you know what).
 * 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.
 * The short-lived Bill and Ted comic book series featured Time Thumb and the Chronological Order, who played this role against the titular heroes.

Fan Fic

 * Netraptor's series of Sonic the Hedgehog fanfiction includes the Time Rippers, an ancient civilization of time travelers.

Film

 * Time Cop.
 * Wilbur Robinson claims to be part of a Time Continuum Task Force when first meeting Lewis in Meet the Robinsons.
 * Cassie in Frequently Asked Questions About Time Travel belongs to one of these organizations.

Literature

 * The Time Patrol from a series of Poul Anderson's stories.
 * 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.
 * Also the Time Patrol in a Tale of Time City by Diana Wynne Jones, although they play a very minor part in the story.
 * 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.
 * 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 Time Commandos in Simon Hawke's Time Wars series.
 * The Time Wardens in Tom Holt's Overtime.
 * 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".
 * They were known to rumor as the Men in Saffron or the No Such Monastery.
 * 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.
 * Time Police agent Josie Bauer in the Callahans Crosstime Saloon story "Have You Heard the One...". She collars the time criminal Al Phee, who's trying to change the past in a minor way to make a profit in the future.
 * Josie revealed that she got the job from her father, a famous science-fiction writer and part-time Time Police agent. At the end of the story it's revealed that her father was in fact Philip Jose Farmer (she let it slip that he was writing a new Riverworld book). In the end, the Traveling Salesman was taken down by the Farmer's Daughter!
 * In JK Rowling's Harry Potter, 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 Time Patrol from Robert Silverberg's Up the Line.
 * In Christopher Stasheff's Warlock 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.
 * The protagonist of the Jack Chalker novel Downtiming the Night Side gets caught up in a full-scale battle over the timeline that turns out to be a second front of The War of Earthly Aggression. One side portrays themselves as time police and their opponents as terrorists. The other side doesn't bother with such niceties.
 * 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.

Live Action TV

 * Voyagers! 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.)
 * 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.
 * Mirai Sentai Timeranger and their American counterparts Power Rangers Time Force.
 * Though they end up in this role, Power Rangers Time Force was originally just a non-time-traveling 30th century elite police force with a suspiciously prophetic name. And a Time Machine Combining Mecha, Time Ship and Humongous Mecha that forms and mans a Cool Gate for time travel. Ya know, just in case.
 * Averted in Power Rangers SPD: one of the Space Police is a time traveler, but he's only trying to Set Right What Once Went Wrong after an alien invasion won; his normal cop duties have nothing to do with time travel.
 * Lost's show runners have described Mrs. Hawking as a "temporal policeman" who prevents Desmond from changing the past during his initial visit to 1996.
 * Not exactly time travel, but in Parallax the Guardians are charged with preventing pollution of the alternate realities.
 * Averted in the various Stargate series, especially SG:1, but still worth mentioning because that setting really should have Time Police. Despite multiple methods of Time Travel in the setting, there are no people or Clock Roaches preventing paradox or enforcing a preferred timeline. Despite Precursors and Sufficiently Advanced Aliens who are totally willing to tell humanity You Are Not Ready on other issues like interstellar travel and certain weapons of war, they've never warned people away from screwing with history. Both good guys and bad guys do it. When people time travel and step on the Butterfly of Doom, the heroes have to make Heroic Sacrifices to Set Right What Once Went Wrong and still put up with a Close Enough Timeline.
 * 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.
 * The Cleaners in Charmed are an organisation dedicated to upholding The Masquerade, though, in carrying this objective out, they are capable of acting as Time Police Officers, by such means as turning back time and even erasing all trace of offenders from existence if they deem it necessary.
 * Fate in the Supernatural episode "My Heart Will Go On".
 * Bernards 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).

Tabletop Games

 * 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.
 * The Guardians of Forever in Genius: The Transgression. The Terminals filled the role before they were removed from the timeline.
 * 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 creepy things living outside Creation).
 * 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.

Video Games

 * 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 War Craft 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 Elder 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,  Time travel is confusing.
 * Furthermore,.
 * 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.
 * Saint Germain from Curse of Darkness too, of course, Epileptic Trees suspect they may be the same person.
 * 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 who is subsequently killed and tries to take over the body of . 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.
 * Final Fantasy XIII-2 features this trope
 * Magical Girl Lyrical Nanoha As 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.

Webcomics

 * 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.
 * The Time Line Authority in TRU-Life Adventures.
 * Gastrophobia: If this page of possibly-canon strips is to believed, then Philia is not an ancient Amazon, but actually an undercover Time Cop posing as an Amazon.
 * Even better. She's Phobia's Kid From the Future, an Amazon and a Time Cop.
 * Used in this Bahala Na strip.

Western Animation
"Fry: I thought your job was to cast the tie-breaking vote in the Senate.
 * Time Squad
 * Gadget Boy's Adventures in History.
 * 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.
 * One episode of Dr. Zitbag's Transylvanian Pet Shop involved the Time Police.
 * 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.

Al Gore: And protecting the time-space continuum. Read your constitution."