San Dimas Time

"The clock in San Dimas is always running."

—Rufus, Bill and Teds Excellent Adventure

San Dimas Time is used when a writer wants to add some against-the-clock tension to a Time Travel story without thinking too hard about how little sense that makes.

As a result, events in two different time periods are shown to happen concurrently, so that people two years in the past only have X minutes to stop the villain from committing some terrible act in the present, even though they should technically have X minutes plus two years to sort it all out. Or perhaps the heroes have only Y minutes to get to their time machine and prevent the villain from doing something thirty years ago, which obviously makes no sense either. (Whatever "our" next year may bring, there is little risk of its world suddenly having experienced a Nazi victory in 1945.)

Alternately, characters traveling to some other time can't come back to the moment they left, but are somehow bound to return to a time, for example, eight minutes after they left if they were gone eight minutes. Can be justified, however, if time travel is of the "travel exactly X time forward/backward" variety. Or perhaps the characters just need to avoid paradoxes, but it's okay to Trick Out Time.

Note that this is different from the Portal to the Past, where a time portal links two eras and allows time on both sides to run at an equal rate, giving the impression that events are running concurrently. This is essentially a portal that sends you X amount of time forward/backwards in time. The main difference is that the Portal to the Past means that the time flow rate on both sides are the same due to both sides being essentially at rest relative to one another (i.e. because of relativity).

If you want to be charitable, you can blame this on the Timey-Wimey Ball.

San Dimas Time is often portrayed using Meanwhile in the Future. (That page, however, specifically focuses on scenes in which the two time periods do not affect one another.)

If the characters are in the past relative to the key events, they might avert this by taking The Slow Path. If they are in the future, they may think they can tell whether or not they are going to have succeeded (by simply reading a newspaper or otherwise) — but perhaps somebody Tricked-Out Time, or maybe there's going to be a Delayed Ripple Effect "after" the villain changed the past.

Note: Please don't duplicate entries between this trope, Meanwhile in the Future, and Portal to the Past.

Examples of San Dimas Time include:

Anime and Manga

• Time Paladin Sakura seems to run on this: Sakura hears on the past her base is being attacked, and comes back to find it wrecked.

Comic Books

• Early Silver Age Legion of Super-Heroes stories are full of such moments, with characters crossing back and forth between the 20th and 30th centuries as if they were just on the other side of town. "Oh no! We're too late!"
  • Subverted in a 1990s Superboy story, when the Legion lose a member in the timestream. The Legionnaires insist they need to find her now, before something happens to her, to which Brainiac 5 replies, "Doesn't anyone realize we're talking about time travel?"
  • In the Legion's tie-in story to Final Crisis, Superboy burns a line across Superboy-Prime's chest, and the same mark appears on the chest of Prime's future counterpart, Time Trapper.
• The DCU's Crisis on Infinite Earths had a wall of anti-matter that moved through the Multiverse slowly destroying all time periods. The future was destroyed first, and the wall encroached backwards through time. Hence the Legion of Super-Heroes (who live in the 30th century) being decimated in an early issue, as well as (more famously) the Flash, Barry Allen, who had made his home in the future. As to why this destructive force went backwards; If it moved forward, nothing after it could stop it. It would cease to exist and you'd have no story.
• Donald Duck time travel generally uses this rule, at least in the sense of characters returning X hours later. One especially convulted example was a story where Donald and his nephews were sent back in time with the help of Gyro Gearloose's time-travelling bathtub to retrieve an atomic bomb accidentally sent back in time to the prehistoric era. During their mission they remain in contact with Gyro through a telephone, and both the past and present timelines run in parallel through the story, including a moment where a time paradox causes the planet Earth to start disappearing as history attempts to correct itself due to the effects of the bomb, and then automatically un-corrects itself after Donald succeeds. Gyro's attempt to explain how the Earth was destroyed and un-destroyed when the bomb went off (even though it didn't) only manages to give Donald and the nephews a headache when they try to wrap their heads around it.
  • Also by Disney, the stories where Mickey Mouse and Goofy (and sometimes other people) travel on Prof. Zapotec's time machine. Example: the duo complains about being called to see the Aztec's proto-soccer right before the finals of a championship, travel, and arrive hours later, where Zapotec has recorded the game and such.
  • Of course, they had a good way to justify this: The machine didn't travel with them, it just sent them to a specific time and location, and in order to avoid confusion they set it to lock on to the same spot after a certain amount of time had passed in both streams. That way, if Mickey and Goofy didn't reappear when Zapotec activated the recall function, he knew that he could try again once the same amount of time had passed. And they knew they had to be there at the exact time he would set the machine to.

Fan Fiction

• In Sailor Moon Zodiac "episodes" 10 and 11, a botched attempt to have the modern Inner senshi quintet observe their past selves leads not only to this but also to a switch between past and present selves that marks the point where the "series" takes a Hotter and Sexier turn.
• The Doctor Who usage of San Dimas Time (see below) is intentionally averted and discussed in Forever Janette by Rich Morris, which features the Fifth Doctor meeting the Master from the Seventh Doctor's timeline.

Films

• In Teenage Mutant Ninja Turtles III, the Time Traveling scepter (in the present) only has enough power to work 5 times in 60 hours, after which it is destroyed, rendering even the fully intact scepter in 1603 Japan useless as well.
• In Bill and Teds Excellent Adventure, Rufus tells the titular twosome that while they can go back in time to research their history project, time will advance normally in their hometown of San Dimas. This is later contradicted in the sequel when they use the time machine to dramatically increase their musical abilities through long practice while only a few moments pass for everyone else. Even the first movie shows the pair coming back to a few seconds before they initially left so that they could have a conversation with themselves, proving that doubles can exist and raising even more questions.
• Subverted in Back to The Future, when Marty realizes he has all the time in the world to save Doc and sets his return trip to several minutes before he left. Over the course of the trilogy, "we can't go back and try again" is due to either a) problems with the time machine, b) risk of messing up one's own past and causing paradoxes, or c) trying again is out of the question when you're already dead. The drama of a time limit is instead provided by the Clock Tower Finale.
  • That's really more of an aversion, as his original plan was to go back to when he left. He never considered going back to several days after.
  • Played straight with Marty's fate should he fail to get his parents together. Rather than being instantly wiped from existence, he slowly begins to fade, suggesting some sort of time limit running concurrently with the past. The photograph he has of his siblings slowly vanishing seems to work this way as well; his brother and sister are older than him, so they begin to fade first.
• The edited version of Highlander II the Quickening replaced "The Planet Zeist" with "The Distant Past" and walked right into this trope.
• Played painfully straight in the first sequel to Halloweentown. Two characters travel back years before the movie, and they have to get back before midnight of that day. While this would seems trivial for people with time travel at their disposal, that does not seem to be the case in this film, with one character commenting: "It's almost twelve and so far we're only at the 1970s." This could be related to the ambiguity of the time stream, making time travel so complex that the characters are lucky to find the right time period at all.
• In a notable scene from Frequency, the bad guy gets his hand blown off with a shotgun, meanwhile his 30 years in the future counterpart is shocked to see that same hand wither away into a nub while strangling his original opponent's son.
  • Later the father, who originally died 30 years ago appears out of nowhere to finish off the bad guy.
    • In Frequency, 1969 time and 2000 time seems to be hooked up only when the ham radio is on—which it is during that scene.
• In Best Defense, Dudley Moore races against the clock to install a cooling system in a tank, just in the nick of time to save Eddie Murphy ... ten years later.
  • In actuality, while the two scenes are intercut to build up dramatic tension, in the "real" timeline the cooling system was there the whole time... it just wasn't shown to the audience until the critical moment, leaving its existence open to doubt until we're actually shown Moore managing to get it included in the design. (This forced, fake tension is simply one of the many reasons this movie features prominently in many people's "worst movie ever" lists.)
• In Austin Powers: The Spy Who Shagged Me Fat Bastard steals Austin's mojo in 1969 when he was still cryogenically frozen. For an unexplained reason, Austin's mojo seems to drain out of him back in 1999 at the same rate Fat Bastard is stealing it in the past.
• The In Name Only festival of shame that is the A Sound of Thunder film overlaps this with a Portal to the Past. The time machine basically opens a single "gate", and while it's open the people that go through are experiencing the past at the same rate that people in the future/present are experiencing it. Part of the climax is that, after everything goes horribly (and predictably) wrong, they have to go back in time to the *same* period and stop the screw up. All this is pretty in synch with various hypothetical methods of time travel. The ways it affects the future... No. Just no.
• Thoroughly averted at the end of Stargate: Continuum, where Cam Mitchell, trying to prevent Ba'al's time travel plot, can't get sent back to the exact moment (the necessary solar flare can't be adjusted, and there won't be another one). So he just waits a few years. Played straighter in the beginning, when Ba'al's changes to the timeline cause the Tok're to disappear one by one as he makes them.

Literature

• Isaac Asimov's The End Of Eternity, in which an organization is able to travel through our time, while maintaining a wholly separate, normal, only-goes-forward-at-one-second-per-second time of their own. This necessitates the inventions of the words "upwhen" and "downwhen" for the time travelers to be able to talk about the past and the future as it exists in "our" time, instead of their own future and past. They refer to this as "physiotime", as it reflects the aging process, not the passage of time.
• Appears in The Prometheus Project by Steve White. The heroes are rushing to keep the villains from completing a time machine that will let them change the past, obliterating the present. They're too late, and the villains activate their machine. Fortunately, the heroes have a faster time machine, which they can use to catch the villains before they reach the past, even if they use the time machine later. Somehow, the past doesn't change as soon as the villains leave.
• In the Xanth series, the time period that the isthmus is connected to all move forward at the same rate at the rate as Xanth. If a Xanthian was to spend a year in The Dark Ages, they would return to Xanth a year later. This spell was done that way so Xanth can trade with other time periods, and the rules of time travel clearly don't require it, as unescorted non-Xanthians end up randomly dislocated in time going in, for security purposes. Technically, nothing stops Xanthians from going to Earth, going back into Xanth, redirecting the isthmus to reconnect to Earth a hour earlier, going back to Earth, and meeting themselves there, and then coming back escorted by themselves so that one of them ends up in the wrong time, but the isthmus connection is incredibly vague about what time period it connects to, and getting somewhere near the right century is amazing, so that would not be possible in practice. (To say nothing of the paradoxes.)
• Works for the villains in Harry Turtledove's The Guns of the South because the time machine only works in increments of 150 years, meaning that because they got the device in 2014, they could travel back to 1864 and no earlier.
• The time travel in H.P. Lovecraft's The Shadow Out of Time seems to work this way. When a member of the Great Race of Yith takes over a human body in the present, the said human's mind in turn goes back in time to the body of the creature that possessed him. The creature then spends several years in the person's body studying the history and culture of the era, and "meanwhile" the person spends equal amount of time in the creature's body, forced to write down all he knows about his own civilization. It's left very unclear if this is just for the sake of convenience, or because of some unknown law of time travel, however.
• In By His Bootstraps by Heinlein, Diktor apparently tried to invoke San Dimas Time on his predecessor self, but was rebuffed by, "How can we waste time when we have this?" So he smoothed it over with fast talk and invocation of authority.
  • It makes some sense here, since the protagonist is essentially every single important character in the story. His own personal clock keeps ticking, meaning things need to happen precisely when they did/will/must happen.
• The whole of the Pendragon series, by D.J. MacHale. Made worse by the fact the Travelers ostensibly arrive exactly when necessary to stop the evil plot, that half of the territories are separated solely by a time difference, and that the villain doesn't have to follow any of the same rules. Massive case of delayed fridge logic.
  • Eventually, the Travelers wise up, realizing that Saint Daine can pretty much undo any repair of a critical point, simply by going back to it.
• In the later books of Stephen King's Dark Tower series (which involves dimensional as well as time travel), there is revealed to be a Keystone Earth in which time always moves forward and the clock is always ticking. Time does not move forward at a constant rate, though, periodically "lurching" forward in relation to the universe in which the majority of the action takes place.
  • This isn't due to Keystone Earth, though; the "lurching forward" is a direct result of Roland's own world having "moved on".
• L. E. Modesitt's Timegod series has this as an explicit rule: a timediver cannot superimpose himself or herself in space and time. So if one screws something up, he can't just go back a few minutes and try again.
• Whenever the protagonist of Octavia Butler's Kindred is dragged back in time to save Rufus, the time that passes in the present before her return is compressed but proportional to how long she spends in the past. When, for example, she spends a few minutes in the past, she disappears in the present for a mere second or two, but when she accidentally leaves her husband in the past, she spends three weeks in the present before going back and learning that her husband has been stranded for over five years.
• The short story "Wikihistory" by Desmond Warzel would seem to operate on San Dimas time; otherwise there would be no suspense regarding AsianAvenger's return.
• Averted hard in Time Travelers Never Die (2009) by Jack McDevitt. The protagonists realize early on that the clock is not always running in San Dimas, and use that fact to prepare for time trips or to bail themselves out of dicey situations.
• Lynne Reid Banks' Indian in The Cupboard series has the titular cupboard's method of time travel actually function this way, with Omri's dad speculating that time is like a corkscrew.

Live Action TV

• In Doctor Who, whenever the Doctor meets another Time Lord, no matter how many trips to various eras each has made, they always seem to both remember past events in the same order. Official universe compendiums have confirmed the Fanon guess that all the TARDISes and everyone traveling in them run on Gallifrey Time, which usually works the same as San Dimas Time, barring Timey-Wimey complications.
  • Thus, if the Doctor experiences event A followed by event B, the Master must also experience event A followed by event B, though the number of years in between might differ considerably for the two. The same applies to the Doctor meeting earlier regenerations.
  • In the Doctor Who Expanded Universe, the BBC Books novel Emotional Chemistry maintains a continuous narrative crossing between three time periods, with four separate means of time travel, and only one point at which one of the characters is "out of sync" with the others.
  • Subverted in "Silence in the Library"/"Forest of the Dead" when the Doctor meets, for the first time, a woman who has known him for years. It happened to be the last time she met him; she dies shortly afterwards. All of her onscreen appearances since have been earlier and earlier in her personal timeline.
    • River Song is a strange type of Inversion, in that she apparently routinely experiences events in the opposite order to The Doctor. (Sometimes they experience events in the same order; otherwise their tradition of comparing diaries would be pointless.) She is aware that the first time she meets The Doctor will be the last time he meets her, which (although it is a Foregone Conclusion for the audience) isn't really necessitated by anything she knows.
    • This same subversion had been done in the previous series, though on a smaller scale. Within their personal timeline, the Doctor and Martha are falling prey to Mr. Saxon's manipulations before they travel to the end of the universe, release the Master and accidentally give him the means to go back in time and establish himself as Mr. Saxon on 21st century Earth.
    • Mel and the Sixth Doctor "first" met each other in a different order, too.
  • In the story "Planet of the Spiders", events take place on 20th Century earth and on a distant planet in the far future, with lots of time/space travel between the two by multiple methods, but somehow all the events happen "in story order" on screen, with no exceptions. The time zones might as well be places.
  • In "Day of the Daleks", the Doctor attributes the "eight minutes in the past = eight minutes in the present" thing to the "Blinovitch Limitation Effect". Mind you, that's his excuse for everything time-related.
  • Interestingly, the few occasions when time-travelers do appear to be out of sync with each other there is generally non-Gallifreyan technology involved (Like Captain Jack's Time Agent wrist band).
  • In the revived series, this trope is required to make the Doctor being the Last of His Kind meaningful. Otherwise, he could run into pre-Timewar Time Lords and Daleks.
    • Well, the entire point of a Time War would seem to be that there is no longer any such thing as a "pre-Timewar Time Lord or Dalek". The participants may remember a time where there was no Time War, but such a time no longer exists. The End of Time seems to support this: Time Lords still exist in the timeline, but they are all locked into their fate. And it's a good thing...
      • Except that the time before the time war can be reached if a malfunction similar to what happened in Rise of the Cybermen were to happen, as seen in the New Series Adventures novel "Prisoner of the Daleks".
  • In one Doctor Who Expanded Universe book, the villainess, a Time Lady, monologues (to the readers; she's alone in the room) that she ought to be going up against the (then-current) Seventh Doctor. She then decides that he'd be too much for her, looks at all thirteen regenerations, and decides to pick on the Fifth instead, even though that's breaking the rules about meeting out of order. He's still too much for her.
  • Note that San Dimas Time is justified in "Pyramids of Mars", when the Doctor has only a narrow window of opportunity to trap Sutekh in the spacetime tunnel. Even if he were to use the TARDIS to return to Earth at a point hours or weeks earlier, he'd only wind up waiting around for the few minutes when Sutekh is in the tunnel, and can therefore be trapped. The fact that he rushes to get it done immediately is more an indication of his excitement-level than fear of wasting precious San Dimas Time.
  • The Doctor always has this situation when on an inter-temporal phone call for obvious reasons.
  • Don't try to make sense of the events of "The Pandorica Opens" and "The Big Bang" without assuming that in the Doctor Who universe, time goes in two directions at once; while the TARDIS can go back and forth in linear time, there's clearly some sort of clock running in a different way, just so The TARDIS exploding causing every star in the universe to simultaneously supernova at every point in time makes sense.
  • This is averted in "A Christmas Carol" where the Doctor spends a lot of time in the past but only little time passes in the present.
  • The opening of "End of Time" plays this completely straight: The Ood warn that "events that have happened are happening now," and the Doctor runs like the blazes back to the TARDIS in order to travel back to the twenty-first century. He's just a bit too late.
• "Future Tense", an episode of Star Trek: Enterprise, averted this trope. When the crew successfully activates the "homing beacon" of a time traveler from the future, the traveler and all his devices are instantly transported home. Captain Archer notes afterwards that once they got the beacon's signal, the other travelers had as much time as they needed to locate it and plan the retrieval.
  • Averted in the Star Trek: Deep Space Nine episode "Time's Orphan", where an 8-year-old Molly O'Brien falls into a time portal, and it closes. When the portal is reactivated, only a few hours have passed on the sending end, but 10 years have passed on Molly's end, where-in she was stranded alone in the wilderness. They choose not to avert the trope again by trying to rescue her at some earlier point in her missing time, because that would mean erasing/killing the Molly in front of them. When she proves violently unable to readjust, they realize she doesn't belong with them any more and try to send her back "home" rather than let her be forced into treatment that amounts to intolerable incarceration for the Wild Child. The trope ends up averted again, when 18-year-old Molly arrives in the past shortly after her younger self. She quickly sends the little girl home before the portal can be closed again, erasing herself but reuniting the O'Briens and their young daughter after all.
• Mighty Morphin Power Rangers; the episode "Wild West Rangers" had Kimberly thrown back to The Wild West, leaving Billy and the remaining Rangers trying to retrieve her. Billy specifically states at one point "It's a paradox. Our World won't change until Kimberly battles the monster in her time", implying the two timelines had become synchronised.
  • The events of Power Rangers Time Force are built on this trope. Ransik goes back in time to the year 2001, and the Rangers follow him a few minutes later. If the Power Rangers universe didn't run on San Dimas Time, the year 3000 would have been instantly altered at that point; there would have been nobody in 2001 to stop him. Later, it's explained that the things the Rangers and Ransik do in 2001 alter the future on a 1:1 ratio. This also explains why some times the Megazord is unavailable due to repair, despite the fact that, by all reasons, Time Force should be able to take whatever time they need to repair the zords before sending them back in time to when they are needed. In the following season, which takes place in 2002, the Crossover has the Rangers contact 3001, a year later from the future timeline.
    • However, Time Force muddles the trope due to it's theme of You Can't Fight Fate Vs. Screw Destiny - Alex knows events that have yet to happen for the Rangers (the potential death of Mr. Collins, the Time Vortex in the finale). That, combined with his own survival and the whole fate tropes, may simply imply that Ransik never changed history. He always was meant to escape from Time Force, and always meant to be stopped. He went to the past and history remained the same - because he was stopped by Time Force. Or alternatively, He went to the past, succeeded, but over a thousand years, the changes he brought were undone, and Time Force was born nonetheless to stop him. This is part of what makes the series interesting, watching fate tropes and time travel tropes work with one another.
• This is somewhat subverted in Journeyman considering that Dan doesn't have any control over when he time-travels and the time he is missing from the present is never proportional to how long he was in the past. However, Dan lives in the 2000s and his fellow time-traveler Livia lives in the 1940s. Their lives appear to be synchronized and they experience their encounters, which occur in various time periods, in the same order.
• The 1950s Flash Gordon TV show:
  • In one episode the main characters had to travel back to the present day to disarm a bomb set to go off in their time...1000 friggin' years later.
  • An especially bad example occurred in an episode where, after superluminal travel sent the protagonist back in time, he was still able to communicate with the base by radio.
• A variation occurs in the fifth season of Lost. The Oceanic 6 leave the Island in 2004, just as the people left on the Island (Sawyer, Juliet, Jin, Miles, Daniel, and Charlotte) start traveling through time, eventually ending up in 1974. When the Oceanic 6 return to the Island in 2007, they are sent back in time to 1977. From everyone's perspective, it has been 3 years since they last met.
  • It's even weirder than that. Entering or exiting the island displaces people permanently in time, slightly backward when entering and slightly forward when exiting. (When exiting to Tunisia, far far forward.) But all the time travel that happens as the island 'skips' follows San Dimas Time to the letter, and even repeated trips to 'near' the same time were in order, despite them flopping around out of order in the larger scope.
  • It's interesting to notice flight 316 got time displaced (Day to night) and only then did the 6 got yanked off it, meaning they were first displaced 'normally' by entering the island, and then traveled through time to finish the Stable Time Loop. So that means when they got back, they had experienced San Dimas Time for one manner of time travel, but right before that they'd been displaced backwards in time, which doesn't work that way. (Fanon has them arriving on 316 at the moment of Jacob's death/nuke detonation, and getting displaced several days backwards from those events so they could make them happen.)
• On Primeval, each Anomaly seems to operate on its own San Dimas Time. If two prehistoric animals enter an Anomaly two hours apart in their time, they'll emerge two hours apart in ours, in the same order as they entered.
• Where In Time Is Carmen Sandiego? imposed a twenty-two minute time limit from the moment that the time-travelling villains stole a historical artifact/landmark to when history would be irreversibly altered. They never twig that they could go back in time to just before the theft in order to stop it, but that would ruin the quiz show a bit.
• In the final episode of Goodnight Sweetheart Gary ends up stuck in the past and writes a message to Ron in the present on the wall of his flat. Rather than the message having been there throughout the intervening decades, it materialises slowly in the present as Gary writes it in 1945.
• Kamen Rider Den-O uses a variation: the DenLiner (and all other time trains) can only travel between two points in time, the "destination" on the Rider Ticket and the point of origin. The show does have a few episodes where the heroes have to get back to the present in time to prevent another crisis.
• In Quantum Leap, from the moment Ziggy locates where and when Sam's leaped.

Tabletop Games

• GURPS Time Travel's standard scenario states that you can only travel to time "windows", about three months apart (and slowly growing farther apart), that are dragged forward as normal time passes. Why this happens, along with what happened at the point the windows initially extended from, is an unexplained mystery. It should be noted, however, that this is only one of several options provided by GURPS Time Travel - it has rules to cover nearly every version of Time Travel on this site (if not more.)
  • In an even better example of the trope, the same GURPS Time Travel setting has relative time rates for time travelers—every 10 days spent in the past equals one day elapsed in the present, so there's no "coming back to the second you left." The rate can change though, if time gets messed with ... so not only is the clock in San Dimas always ticking, it can change speed!
• The old Teenage Mutant Ninja Turtles supplement "Transdimensional TMNT" had a suprisingly complex justification for San Dimas Time that involved the time stream coiling around itself in predictable patterns and jumping from coil to coil where they touched.

Video Games

• RTS game Achron involves this, though more for gameplay purposes. The present is always moving, and any viewing of the past or future will by default involve the player moving through time at the same speed as the present. Changing any specific point in time also becomes harder as the present moves away from it, for balance reasons. However, the changes made in the past are periodically propagated faster than the player can go by timewaves, and the player can control their rate of movement through time. For example, fast forwarding will cause one to catch up to the present eventually, propagating changes along the way.
  • To put it simply, if somebody else changes the past, you really do only have X amount of time to get back into the past yourself (or send a time-traveling unit back) and prevent that change from becoming irrevocable. But in the contextual structure given for time travel, this makes perfect sense.
• The video game Shadow of Destiny uses this. Since the time in the present is counting down to the time of your inevitable death, it puts a time limit on all the puzzles you need to solve in the past, even though San Dimas Time doesn't really make much sense.* The Journeyman Project series of computer games were all dependent on this trope. In those games, whenever something in the past is altered, it creates "temporal ripples" that take a certain amount of San Dimas Time to "reach" the present.
• The Time Gates of Chrono Trigger are Portals To The Past, which imposes the same kind of things San Dimas Time does with justification. However, later in the game you acquire a Time Machine, and travel with it still inexplicably works the same way. Interestingly, Fanon holds that the Time Machine is based on the preexisting portals between time periods. Here's the relevant article.
• Handwaved in Where in Time is Carmen Sandiego? where it's state that legal time travel has to be cleared with a federal agency, and thus each Chrono-Skimmer will return to the present after a certain number of hours of machine use. Of course, the villains have no such problem.
• Every time you hear the words "Oh no! WE'VE GOT TO HURRY!" in Sonic the Hedgehog 2006, take a shot. Seriously, just shoot yourself in the head. Interestingly Sonic actually ends up being too late, and does simply just hop back in time a few minutes to resolve the issue.
  • There was a level timer in Sonic the Hedgehog CD, too, though there were only three times you could go to.
• The mandatory surfing stages in Mario's Time Machine for the Super NES are timed. As The Angry Video Game Nerd said, "AND HOW IRONIC IS THAT, THAT THERE'S A TIME LIMIT TO GO BACK IN TIME?!" In this case, though, the only downside to letting the clock run out is that you get less bonus points.
• Day of the Tentacle seems to utilize this trope to an extent: The three main characters are stuck in the present, 200 years in the past, and 200 years in the future respectively, and events that happen in the past affect the characters in the future. For example, one character in the past can convince George Washington to cut down a tree in the yard, causing the tree to vanish into a stump in future timelines and another character (who is stuck to the tree by her underwear) to fall on her ass.
• A version that actually makes sense in Chronotron - in one level, your time machine is on a lever controlled by an inaccessible button with a bomb on it. You must complete whatever you need to do before the bomb explodes, the lever goes away and your time machine falls into oblivion. And no, you can't just arrive earlier, because it doesn't work that way.
• Justified and used as a plot point in The Legend of Zelda Oracle of Ages, as the Big Bad has stopped time at high noon around the construction site for the final dungeon. No matter how quickly you progress, you can't stop the tower from being built.
  • However, rather than appear completed instantaneously to your present self, the tower being built in the past progresses in increments as you progress through the plot.
• The high concept of Spider-Man Edge of Time: Actions in the present alter the future, but they do so "simultaneously" in relation to the Portal to the Past; if Spider-Man can't stop the giant robot from ever being built "before" it kills Spider-Man 2099, then it's too late. Miguel Handwaves this as a side-effect of the permenant connection between the two time periods; Peter Lampshades that it still doesn't make any sense.
• In the Rare NES game Time Lord, the clock is always running in 2999, and you have one year to Save the World by traveling through four historical time zones.

Web Comics

• Averted, but referenced, in Superosity: because there is no San Dimas Time, the characters aged while time travelling but returned to The Present a millisecond later, the characters worry that their birthdays might have changed to compensate.
  • Superosity author Chris Crosby has the same problem in another series he writes, Wicked Powered. The characters are some kind of chrono-police, and they jump back in forth between time limits, such as being required to leave in mid-mission to another mission because of presumable San Dimas Time, yet having the option to travel back to any point in their own timeline to correct the myriad of mistakes they make (such as blowing up the earth, among others)
• Played with here in The Whiteboard.
• Almost justified in Sluggy Freelance. At the end of The Storm Breaker Saga, it's revealed the time travelling in this and previous stories has worked as follows: First, the demon K'Z'K is blast into the past accidentally, taking with him Gwynn's soul, leaving Gwynn's body in a coma in the present. He "cuts a noticeable trail" through time and space. Next, Riff's malfunctioning time machine sends Zoe and Torg into the past along that same trail (though it seems they actually arrive somewhat after K'Z'K, which would probably be a blatant example of this trope). They fight and temporarily destroy K'Z'K in that time, whereupon Gwynn's soul returns through time to her body to the present. In the present, Riff and Dr. Schlock have been trying to figure out a way to go after Torg and Zoe for some time, not knowing what time they went to. When Gwynn regains consciousness, she is able to report seeing them, and Riff travels along the trail her soul left, thus explaining why he only arrives in the time after Torg and Zoe have defeated K'Z'K. The question that remains unanswered, besides of why Torg and Zoe don't appear in exactly the same time as K'Z'K, is: why didn't Gwynn's soul return to the time when it left her? One theory is that Gwynn's soul had to maintain a chronological parallel with the physical component.
• Homestuck calls this circumstantial simultaneity. It expresses how two events can happen "at the same time", even if they're not happening at the same time or if the events are in different timelines altogether.
  • An example of this in action is the big End of Act 5 Flash, [S] Cascade, which involves no less than four different chronologies happening "simultaneously". For example, the villain destroys a universe from the outside, shortly after that universe was created. At the "same time", his past self in the future of the same universe is trying to escape from it's destruction. This universe then explodes at the "same time" as another universe, even though the second universe had existed before the first one.
  • In Act 6 two characters are given a chat program that let's the communicate with two other characters in the distant past. The program is basically designed to run on San Dimas Time.
• During The Packrat's 2011 time travel Story Arc, the titular character spends a year in the past. In his own time, he is actually absent for that year. Kind of justified in that his time machine keytar can only jump whole years.

Western Animation

• Happened in an episode of the Men in Black Animated Adaptation, where Jay and Kay have to travel back in time to the wild west when an energy-absorbing alien who's about to cause havoc in modern time was still vulnerable. According to some Techno Babble, the past and present move at the same rate, so Jay and Kay only have a few hours to finish the job over a century ago or else the alien will destroy a city, which theoretically wouldn't revert to normal even after the alien was killed in its larval stage over a hundred years ago. Nevertheless, when Jay and Kay find themselves running out of time and in need of backup in the past, they can use a device (which only travels through time at the speed of regular time) that will remain dormant until the corresponding present (as opposed to the present from the beginning of the episode) and then relay a message to the rest of the team describing the problem and requesting backup.
• The Jetsons Meet The Flintstones.
• In the DuckTales (1987) episode "Time is Money", the characters appear to have at least a vague awareness that Time Travel doesn't work this way given that they discuss going back in time to change events that occurred during the episode. They never do this, however, and past time periods do appear to be synchronized with the present.
• An episode of Superfriends had Aquaman and a couple others trapped in the distant past. He decides to bury his communicator under the future site of the Hall of Justice and set the emergency signal. The signal isn't detected until after his and several other teams get trapped in different eras and the rest of the team has begun looking for them.
  • Another episode had the Superfriends travelling back in time to stop time-travelling villains from changing their origins. Flash arrives on Paradise Island in time for the tournament where Diana won the right to travel to the world of men. "I hope I'm not too late!" Flash exclaims. Well, Barry, if you are, you could just run back another three hours into the past and try again, couldn't you?
• Completely thrown out the window in the Justice League Unlimited episodes The Once and Future Thing. The timeline was described as becoming fluid, with different time periods merging together because of how much it was screwed up.
• An episode of Biker Mice From Mars had the Big Bad (probably some kind of alien named after cheese, but who cares?) travelling into the past to demolish the city by digging giant holes where it would be built. Whenever he finished digging a hole, it would appear at the same time in the future in the middle of the city. Then again, what did you expect from a show about biker mice from Mars?
• This trope is integral to the second season of Beast Wars. So the Planet Buster is blown up over prehistoric Earth, unleashing the Quantum wave. Half a season later, the Transformers realize the Quantum Wave travels in space and time. It should reach Cybertron sometimes during the time period they come from (24th Century). The wave in fact hits right about the same length of time after the Maximals left their century as they have spent in the past, making our first instance of this. Now, the Tripredacus Council sends an agent back in the past, to assist the Maximals to apprehend the Predacons, figuring out the time period from the Quantum Wave. Now, logically the agent should arrive when the Planet Buster detonated, well before the Maximals even realized the Quantum Wave traveled in time, retconning the second season? No. He shows up in the past several months after the planet buster detonated, showing both time lines are clearly running in parallel.

Real Life

• In one proposed mechanism of time-travel, one end of a wormhole is flown around for a bit at near-lightspeed, which makes it 'age' slower than the other end due to time dilation. Stepping through the stationary end therefore brings you back in time. But both ends of the wormhole still 'age' in normal time, so if you walked through, spent 5 minutes in the past and then walked back, 5 minutes would have passed in the future as well, assuming both ends are stationary when you do the actual time travelling. Otherwise, it starts getting confusing...
  • If you think about it, this wormhole is really a Portal to the Past. If you were to walk into the future end, and then go back to it without using the wormhole again, you just might meet yourself before you went inside (depending on space and time differential between the ends).
  • In short, the clock is always running in San Dimas, but it's subject to time dilation, and isn't guaranteed to run at a constant speed.