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[[quoteright:350:[[Series/StarTrekTheOriginalSeries http://static.tvtropes.org/pmwiki/pub/images/rsz_wchter_der_ewigkeit_2267_2253.jpg]]]]

Who needs a SteamPunk TimeMachine or [[Series/DoctorWho alien police boxes]]? Certainly not someone with a portal to the past. It might be the result of eddies in the time stream or [[AWizardDidIt a magic spell,]] or it might remain unexplained in a MagicRealism style. However it happened, the portal is your two-way ticket to time travelling fun.

The portal is usually synched up with the modern day, so that an hour on ''this'' side is equivalent to an hour on ''that'' side, thus allowing relationships to develop and [[JustifiedTrope justifying]] the existence of SanDimasTime. However, if the story is a little more tragic then time might [[YearInsideHourOutside move faster on one side]], making the character on that side age much faster.

If a melancholy ending or moment of tension is required, a PortalSlam may come into play.

See also MeanwhileInTheFuture, SanDimasTime, PortalDoor, OurTimeTravelIsDifferent.

'''Note''': Please don't duplicate entries between this trope, MeanwhileInTheFuture, and SanDimasTime.

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!!Examples

[[foldercontrol]]

[[folder:Anime and Manga]]
* ''Manga/InuYasha'': The Bone Eaters' Well connects the two time periods in sync, so that time flows at the same rate on both sides.
* In ''Manga/ShinobiLife'' the portal is in a lake in [[{{Ninja}} Kagetora's]] time, and connects to one above a skyscraper in the modern day. We later learn that [[spoiler: there is another portal from the modern day to the future, which Beni's mother often used]].
* In ''Manga/ThermaeRomae'' there is a portal between a modern day bathhouse in Japan and a spa in Ancient Rome.
[[/folder]]

[[folder:Film]]
* The TimeViewer in ''Film/DejaVu'' is permanently set to look at the past about 5 days earlier, and this interval cannot be changed, because they've just accidentally stumbled upon the 'future end' of the type of wormhole described under Real Life below. Oddly, no one thinks of looking through the 'past end' to see the future.
* In ''{{Frequency}}'', apparently because of the Aurora Borealis, a ham radio set sends and receives signals between the present and thirty years ago.
* In the 1999 film ''Film/TheLoveLetter'', a modern-day American man finds letters written in the 1860s hidden in his antique desk. He fancifully responds to these letters by routing his own through a pre-Civil War post office, where they are mysteriously transported back in time to be delivered to the original writer (a previous owner of the desk), who then replies by hiding 'new' letters.
* ''Film/TheSmurfs'':
** ''The Smurfs'' has a portal that opens within a waterfall in the Smurf forest OnceInABlueMoon that transports whoever enters it through time and space...in the movie's case, into modern-day [[BigApplesauce New York City]].
** Gargamel opens up one near the Eiffel Tower in Paris in ''Film/TheSmurfs2'' in order to kidnap Smurfette, but since it's too small for him to go through, he sends Vexy through the portal to get her.
[[/folder]]

[[folder:Literature]]
* ''Literature/MissPeregrinesHomeForPeculiarChildren'' has a cave near the town in which the main story takes place.
* The book ''The Daughter of the Regiment'' had a tiny time portal through which a boy watched a girl in the past grow up.
* The ''{{Nightside}}'' books have Timeslips, leading to both the past and possible futures, depending which one you stumble into.
* Stone circles work this way in the ''Literature/{{Outlander}}'' book series-- but travel is really only ever something like "safe" if you have a certain assortment of precious stones with you, and it's got a very high fatality rate. (And as the heroine muses, the separation in the various times is roughly 200 years... just like in the folktales.)
* Julian May's ''Literature/SagaOfTheExiles'' has a portal to six million BC, initially only one-way.
* First, Second, and Third Earth (past, present and future) in the ''{{Pendragon}}'' series are linked by the PortalNetwork, and employ SanDimasTime.
* In the short story "[[http://www.lexal.net/scifi/scifiction/classics/classics_archive/young2/young21.html Dandelion Girl]]", a person claiming to be a time traveler explains that time machines default to this mode, with their destination time advancing at the same rate as their source time unless you deliberately choose to keep resetting the destination back. Whether that person is telling the truth, however, is a rather important question...
* The {{Cool Gate}}s of the 'Literature/MorgaineCycle'' can do this as a side effect of their PortalNetwork function. Unfortunately, anyone who uses them to create a TimeParadox triggers a civilization destroying TimeCrash, which is why Morgaine is on her quest to destroy all the gates.
* The ''Literature/TimeScout'' series by Robert Asprin is based around an ever growing network of periodicly opening portals, each of which is tied to a particular spot and reaches back a fixed length of time (which varies from portal to portal).
* MichaelCrichton's ''Literature/{{Timeline}}'' has a wormhole that was accidentally discovered during a teleportation experiment. The wormholes leads to the past, specifically, to France in 1357. The events in the present and the past are synced, although it's also implied that YouAlreadyChangedThePast (i.e. no changes in the timeline are possible). When the quantum mirror is damaged by a grenade, the lead technician fears the connection may be lost for good, as they have no idea how the link works.
* The Great Door of Time in ''Literature/SeptimusHeap''.
* In Creator/StephenKing's story "Literature/TheLangoliers," a "rip in time" causes all the passengers not asleep to disappear from a jet, while the plane and the sleeping passengers are sent back "a couple of minutes." The survivors must find out how to get back, and do so asleep, otherwise they too will disappear. They wind up a couple of minutes in the future but are able to sync up with normal time when it arrives.
* In Literature/ElevenTwentyTwoSixtyThree, there is a stable portal in the back of a diner which leads to 1958. Interestingly, while time goes forward in the present, the other end always goes to the same second in 1958, effectively resetting on subsequent visits.
* Lynne Reid Banks' ''Literature/TheIndianInTheCupboard'' series has the magic of time travel function this way, with Omri's dad speculating that time is spiral in shape, and you can "drop through a certain number of layers" to get to a point in the past, but if you then take TheSlowPath (and live long enough) you eventually arrive back where you started. When someone is time-traveling using the magic, their present-day self is unconscious, so they have to be safely hidden and get back in time for nothing to go wrong (one side character is thought to have died as a toy-sized figure in what was, to her, the future because the people in her own time buried her seemingly dead body, and Omri's mother gets a nasty scare when she finds her husband and sons in the car looking like they somehow suffered carbon monoxide poisoning).
* The time doors in ''Literature/ChronoHustle'' can travel to multiple different times and places, but the time syncs up between all of them, so a day passing in one time equals a day passing in another.
[[/folder]]

[[folder:Live Action TV]]
* ''QuantumLeap'' worked rather like this from Al's point of view - the door that took him to Sam was synced up so that if Al went away to do something for five minutes, Sam would have to wait out those five minutes before he returned.
** One episode had changes that took place instantaneously from Sam and Al's viewpoint. Specificaly, the episode had Sam and Al switch places due to a lightning strike. Mistaking this for a radiation spike, the Quantum Leap computer blocked the door to the holo-room. So Al had to send a letter to their future colleagues with a request to deliver at a specific point in time (see ''Film/BackToTheFuture''). As soon as he dropped the letter into the mailbox, the door opened.
** Interestingly, this synchronization only applied after Sam arrived, replacing someone. The leaps themselves were instantaneous from Sam's (and the audience's) point of view, but appeared to take a random amount of time in the future, often several days.
* The whole ''Series/MirrorMirror'' New Zealand TV series was built upon this trope.
* The comedy GoodnightSweetheart had the main character walking down an old cobbled street that would take him - and only him - back in time, where he fell in love with a World War II barmaid and committed time-travelling bigamy. Turns out it was due to the "harmonics" of time and other characters could time travel but none of the main characters.
* The ''Series/DoctorWho'' story "The Girl In The Fireplace" had a malfunctioning portal that which allowed time to [[YearInsideHourOutside seemingly run faster]] on one side and not the other. There were other portals that were more consistent, however.
* Hordes of these show up in ''{{Primeval}}'' where they make up the shows premise. They're called Time Anomalies by the cast.
* ''Franchise/StarTrek''
** ''Series/{{Star Trek|The Original Series}}'' had the Guardian of Forever in the original series episode "The City on the Edge of Forever". Kirk apparently assumed that the Guardian worked on SanDimasTime (leaving orders for the rest of the landing party to go back through the portal when Scotty decided he'd waited long enough), but it turned out not to (when he returned, Scotty said that he'd only "left a moment ago").
** An early ''Series/StarTrekVoyager'' episode had Harry Kim discover a wormhole that led back to the Alpha Quadrant. Only trouble was, not only did it come out in Romulan territory, but [[spoiler:Romulan territory twenty years earlier]]. May be TruthInTelevision: see RealLife below.
** ''Series/StarTrekDeepSpaceNine'' episode "Time's Child" had O'Brien's daughter Molly fall through one. They managed to pull her back, but their instruments were a bit off and they pulled her out ten years after she had gone through.
* [=TRAX=], the time machine of ''Series/TimeTrax'' could only send its subjects approximately 200 years into the past.
** The show is inconsistent with its time travel. Some episodes claim that it's not actually time travel. They're just traveling to a parallel dimension that is 200 years behind. Thus, any changes to this timeline will not affect "their" present. Other episodes have the protagonist communicate with the future by placing ads in newspapers, something that wouldn't make sense if this was a parallel world.
* ''Series/KamenRiderKiva'': a door within Castle Dran can send someone into the past, but many of the details are left vague. What's shown is that the door sends one back 22 years, and they remain there until they achieve the goal they sought to accomplish in the first place. In other words, time travel as the plot requires.
* The ''Series/RedDwarf'' episode "Stasis Leak" revolves around one of these. Lister wants to use it to save Kochanski; Rimmer wants to use it to save himself.
[[/folder]]

[[folder:Tabletop RPGs]]
* The game ''TableTopGame/FengShui'' takes place in four eras (AD 69, 1850, 1996 and 2056) as well as the timeless realm outside of time, the Netherworld. These four eras are connected by various portals, and are synchronized to each other.
[[/folder]]

[[folder:Video Games]]
* ''VideoGame/ChronoTrigger'' has a whole bunch of these portals going to multiple "locations" in time, and always in sync. This doesn't explain, however, why the Epoch itself is on SanDimasTime.
* ''MarioAndLuigiPartnersInTime'' had time portals popping up all over the place, leading to different areas in the same time period.
* The Caverns of Time in ''VideoGame/WorldOfWarcraft'' are all about this. It would of course be difficult, incredibly confusing, and quite pointless to do it in any other fashion.
* The protagonist of ''TimeHollow'' has the ability to open these using his "Hollow Pen". [[spoiler: It turns out that anyone who steps through them will be removed from the flow of time, stopping them from aging from then on.]]
* The ''StarOcean'' series has a planet specifically dedicated to a PortalToThePast: planet [[WhatDoYouMeanItsNotSymbolic Styx]]. It's used about thirty minutes into ''StarOceanFirstDeparture'', mentioned in ''[[VideoGame/StarOceanTheSecondStory The Second Story]]'', and used in ''[[StarOcean3 Till The End Of Time]]'' to [[spoiler: contact the 4th dimensional beings that observe our MMO universe. That's right! We're all game characters that got ''pissed'' and came ''out of the computer'' to tell our creators to ''knock it off''.]]
* Portals to both past and future figure prominently in ''Videogame/DarkFall2LightsOut''.
* ''Videogame/TheLegendOfZelda'':
** ''VideoGame/TheLegendOfZeldaTwilightPrincess'' has precisely one of these in the game. The door to the Temple of Time sends you to the interior of the temple as it was in the past. The entry hall is even [[ContinuityNod replicated nearly exactly]] from the version in ''VideoGame/TheLegendOfZeldaOcarinaOfTime'', including the same background music.
** The two Gates of Time in ''VideoGame/TheLegendOfZeldaSkywardSword'': one in the Temple of Time (appropriately), the other in the [[spoiler:Sealed Temple]].
** Until it gets upgraded, the Harp of Ages in ''VideoGame/TheLegendOfZeldaOracleOfAges'' can only activate dormant time portals in fixed locations. Once upgraded though, Link no longer needs to locate a portal to travel through time but can do it anywhere except in the graveyard or near the black tower.
* The ''VideoGame/FinalFantasyXI'' expansion ''Wings of the Goddess'' introduced Cavernous Maws, intimidating-looking examples of this trope that connect present Vana'diel with the Crystal War era.
* In ''CityOfHeroes'', the Midnight Squad uses one to access the ancient Roman town of Cimerora.
* Appears in ''{{Okami}}'', causing the protagonist to have to reenact a famous battle -- in which the protagonist died.
* There are a few user-made maps in ''VideoGame/{{Portal2}}'' that use Hammer's world portal function like this. Most of them are just proof of concept levels, though.
* [[http://steamcommunity.com/sharedfiles/filedetails/?id=159356833 This]] fan puzzle for ''VideoGame/{{Portal 2}}'' features one. Apparently the portal gun can transport you across time as well as space.
[[/folder]]

[[folder:Webcomics]]
* {{Conversed}} in ''Webcomic/TalesFromThePit'', causing another lost day of R&D productivity as the Pit enters a heated discussion about the best way to make money if you had a TimeMachine that can only go back exactly 24 hours.
* This appears to happen briefly within the first couple strips of Webcomic/GirlGenius. The story hasn't caught up the wherever the future end was, though, so we can't be sure.
[[/folder]]

[[folder:Western Animation]]
* The ''{{Superfriends}}'' episode "The Time Trap" uses this trope.
* WesternAnimation/JackieChanAdventures had this happened three times all involving the character of Jade. In each instant one could only travel in one direction through time, meaning they had to provide their own "return ticket."
* ''WesternAnimation/KimPossible'':
** The [[TimeMachine Tempus Simia]] allowed time travel to the past and to the future via portals it made when activated.
** The wristwatch [[TimeMachine time machines]] also created portals.
* A veritable part of the landscape in SamuraiJack: almost every episode features a new time portal that's [[StatusQuoIsGod invariably]] destroyed in the end to spite [[FishOutOfTemporalWater our poor samurai hero]].
[[/folder]]

[[folder:Real Life]]
* Physicist Kip Thorne discovered, using general relativity, a possible way to use wormholes to create links between time periods. The time-synching phenomenon is explained by the fact that after the wormhole/time portal is set up, both sides will be essentially at rest with one another, meaning that time must flow at the same rate on both sides.
[[/folder]]

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