Portal to the Past

Who needs a Steampunk Time Machine or alien police boxes? Certainly not someone with a portal to the past. It might be the result of a eddies in the time stream or a magic spell, or it might remain unexplained in a Magic Realism 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 justifying the existence of San Dimas Time. However, if the story is a little more tragic then time might 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 Portal Slam may come into play.

See also Meanwhile in The Future, San Dimas Time, Portal Door, Our Time Travel Is Different.

Note: Please don't duplicate entries between this trope, Meanwhile in The Future, and San Dimas Time.

Anime and 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 Shinobi Life the portal is in a lake in Kagetora's time, and connects to one above a skyscraper in the modern day. We later learn that.

Film

 * The Time Viewer in Deja Vu 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 1998 film The Love Letter, an 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.
 * The Smurfs has a portal that opens within a waterfall in the Smurf forest Once in A Blue Moon that transports whoever enters it through time and space...in the movie's case, into modern-day New York City.

Literature

 * Miss Peregrines Home for Peculiar Children 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 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 Saga of the Exiles 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 Portal Network, and employ San Dimas Time.
 * In the short story 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 Gates of the Morgaine Cycle can do this as a side effect of their Portal Network function. Unfortunately, anyone who uses them to create a Time Paradox triggers a civilization destroying Time Crash, which is why Morgaine is on her quest to destroy all the gates.
 * The Time Scout 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).
 * Michael Crichton's 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 You Have Already Changed The Past (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 Septimus Heap.

Live Action TV

 * Quantum Leap 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 Back to The Future). As soon as he dropped the letter into the mailbox, the door opened.
 * The whole Mirror Mirror New Zealand TV series was built upon this trope.
 * The comedy Goodnight Sweetheart 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 Doctor Who story "The Girl In The Fireplace" had a malfunctioning portal that which allowed time to 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.
 * Star Trek 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 San Dimas Time (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").
 * TRAX, the time machine of Time Trax 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.
 * Kamen Rider Kiva: 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 Red Dwarf episode "Stasis Leak" revolves around one of these. Lister wants to use it to save Kochanski; Rimmer wants to use it to save himself.

Tabletop RP Gs

 * The game Feng Shui 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.

Video Games

 * Chrono Trigger 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 San Dimas Time.
 * Mario & Luigi: Partners In Time had time portals popping up all over the place, leading to different areas in the same time period.
 * The Caverns of Time in World of Warcraft 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 Time Hollow has the ability to open these using his "Hollow Pen".
 * The Star Ocean series has a planet specifically dedicated to a Portal to The Past: planet Styx. It's used about thirty minutes into Star Ocean First Departure, mentioned in The Second Story, and used in Till The End Of Time to
 * Portals to both past and future figure prominently in Darkfall 2: Lights Out.
 * The Legend of Zelda:
 * The Legend of Zelda Twilight Princess 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 replicated nearly exactly from the version in The Legend of Zelda Ocarina of Time, including the same background music.
 * The Gate of Time in The Legend of Zelda Skyward Sword, located in the.
 * The Final Fantasy XI 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 City of Heroes, 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.

Webcomics

 * Conversed in Tales From the Pit, 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 Time Machine that can only go back exactly 24 hours.
 * This appears to happen briefly within the first couple strips of Girl Genius. The story hasn't caught up the wherever the future end was, though, so we can't be sure.

Western Animation

 * The Superfriends episode "The Time Trap" uses this trope.
 * Jackie Chan Adventures 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."
 * Kim Possible:
 * The Tempus Simia allowed time travel to the past and to the future via portals it made when activated.
 * The wristwatch time machines also created portals.
 * A veritable part of the landscape in Samurai Jack: almost every episode features a new time portal that's invariably destroyed in the end to spite our poor samurai hero.

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.