A Tale of Time City

A 1987 novel by Diana Wynne Jones.

It's 1939 in London, and Vivian is being 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 Place Beyond Time from which scholars observe the whole of human history, from the earliest days of mankind to the 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?

Tropes include:

 * Artificial Human: Elio. He's called an "Android," but he's not a Ridiculously Human Robot--he's made of flesh and bone, just different.
 * Beethoven Was an Alien Spy:
 * Big Eater: We really don't know where Sam puts it all.
 * Blitz Evacuees: Vivian.
 * Crystal Spires and Togas: Time City.
 * : Cousin Vivian, in the end.
 * Eternal Love: Faber John and the Time Lady.
 * Fan of the Past: Elio loves movies, especially older ones like Snow White. This gives Vivian something to bond with him over.
 * Fish Out of Temporal Water
 * Food Porn: Especially of the butter-pies.
 * Historical In-Joke: Some of the ripples in time caused by  are actual historical events, such as the eruption of Pompeii.
 * In the Future, Humans Will Be One Race: Sort of. The Lee family seems to be an interesting mix of traits from several different ethnicities; Chinese is the only confirmed one.
 * King in The Mountain: Faber John sleeps under Time City.
 * Living Memory: The Time Ghosts.
 * Oh My Gods: "Great Time!"
 * One Steve Limit: There is quite the abundance of Vivians running around this story. In addition to Vivian the protagonist, she's imitating a Cousin Vivian, whose father is named Viv (short for guess what). And, of course, the Time Lady's real name is Vivian.
 * Place Beyond Time: Time City, naturally.
 * Scavenger World:
 * Shout-Out: Elio, who enjoys movies, at one point starts watching "an animated film about rabbits."
 * Sufficiently Advanced Technology
 * The Spock: Elio does not have very strong emotions, normally, but he does have a slight sense of humor.
 * Rubber Band History: 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.
 * Time Crash: What the protagonists want to prevent.
 * Timey-Wimey Ball: The time-travel almost--but not quite--makes sense if you assume that time is two-dimensional.
 * Trademark Favorite Food: Sam loves butter-pie so much, he can eat over a hundred of them in a single sitting!
 * World War II: Vivian's home era.
 * You Can't Go Home Again: Vivian's home era being Unstable (i.e., shifting in time) means it's difficult for her to go home.
 * Zeerust: The oldest portions of Time City bear a slickly futuristic look. The newest portions have gone for a more classical appearance.