A 1987 novel by Creator/DianaWynneJones.

It's 1939 in London, and Vivian is being [[BlitzEvacuees sent out of her home city to escape the bombings.]] But while she's looking for her Cousin Marty on the train platform, two boys swoop down, abduct her, and ferret her off to their homeland--Time City, a PlaceBeyondTime from which scholars observe the whole of human history, from the earliest days of mankind to the [[WagonTrainToTheStars Depopulation of Earth]] in Hundred and Seventeen Century. Due to her name--Vivian Smith--they think she's really Faber Vivian, one of the city's founders. And, in fact, one of the people they think is slowly destroying their city! The boys, Jonathan and Sam, quickly realize the error of their ways. One problem--Vivian is from an Unstable Era, and ''no one'' is allowed to come out of an Unstable Era, lest the whole of human history be altered!

Now they must pass Vivian off as their cousin, and she must adjust to Time City life. But the city is still falling apart! Will she, Sam, and Jonathan be able to figure out what's causing the city's destruction before it's too late?
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!! Tropes include:

* ArtificialHuman: Elio. He's called an "Android," but he's not a RidiculouslyHumanRobot--he's made of flesh and bone, just different.
* BeethovenWasAnAlienSpy: [[spoiler: Leonardo da Vinci was a megalomaniacal student from the sixty-sixth century, actually. Maybe.]]
* BigEater: We ''really'' don't know where Sam puts it all.
* BlitzEvacuees: Vivian.
* CrystalSpiresAndTogas: Time City.
* DaddysLittleVillain: [[spoiler:Cousin Vivian, in the end.]]
* DontEatAndSwim: Jonathan and Sam's parents hold them to this during the picnic at the lake.
* EternalLove: Faber John and the Time Lady.
* FanOfThePast: Elio loves movies, especially older ones like ''Disney/SnowWhite.'' This gives Vivian something to bond with him over.
* FishOutOfTemporalWater
* FoodPorn: Especially of the butter-pies.
* HistoricalInJoke: Some of the ripples in time caused by [[spoiler: the upheaval of Time City]] are actual historical events, such as the eruption of Pompeii.
* InTheFutureHumansWillBeOneRace: ''Sort'' of. The Lee family seems to be an interesting mix of traits from several different ethnicities; Chinese is the only confirmed one.
* KingInTheMountain: Faber John sleeps under Time City. [[spoiler: Except it's actually his wife. He himself is very much awake.]]
* LivingMemory: The Time Ghosts.
* MagicFromTechnology
* OhMyGods: "Great Time!"
* OneSteveLimit: There is quite the abundance of Vivians running around this story. In addition to Vivian the protagonist, she's imitating a Cousin Vivian [[spoiler: who appears later--no one seems to care by then]], whose father is named Viv (short for guess what). And, of course, the Time Lady's real name is Vivian.
* PlaceBeyondTime: Time City, naturally.
* ScavengerWorld: [[spoiler: The Age of Gold, where humanity still keeps certain aspects of technology, but has largely reverted to a pastoral way of life--with lots of raiders.]]
* ShoutOut:
** Dr. Wilander's name is a shout-out to Literature/WaylandTheSmith, [[spoiler:and thus a hint toward his connection to Faber John]].
** Elio, who enjoys movies, at one point starts watching "[[WesternAnimation/WatershipDown an animated film about rabbits.]]"
* TheSpock: Elio does not have very strong emotions, normally, but he does have a slight sense of humor. [[spoiler: And when he realizes he's failed Jonathan, Vivian, and Sam, he comes to understand despair.]]
* RubberBandHistory: The Fixed Eras are explicitly like this--while the Unfixed Eras change and warp, Fixed Eras stay the same so long as not ''too'' much gets weird in the Unfixed ones. [[spoiler: This all changes when Time City starts its cycle again.]]
* TimeCrash: What the protagonists want to prevent.
* TimeyWimeyBall: The time-travel almost--but not quite--makes sense if you assume that time is two-dimensional.
* TrademarkFavoriteFood: Sam loves butter-pie so much, he can eat over ''a hundred'' of them in a single sitting!
* YouCantGoHomeAgain: Vivian's home era being Unstable (i.e., shifting in time) means it's difficult for her to go home. [[spoiler: There's also the fact that she's a walking time disrupter. In the end, when history re-settles into new eras, she still can't go home, but Faber John agrees to bring her parents to Time City.]]
* {{Zeerust}}: The oldest portions of Time City bear a slickly futuristic look. The newest portions have gone for a more classical appearance.
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