Pick a historical event at random. Now, how many years ago did that event happen? Probably a completely random number of years ago. It might be close to 100 or 150 years, but if so, it's a coincidence. It's usually quite random.
Now pick a fictional historical event. How long ago was that, compared to when the characters first mentioned it? Usually, 20 years, or 50 years, or some number divisible by 5, and if not, then 3 years, 7 years, 10 years, or 12 years.
Writers are busy people. Not all of them have the time to think up random dates for each notable event. Some writers downplay this trope by making it "roughly" a round number of years ago, but they won't actually say how many years ago it really was. If a character in the story casually mentions how long ago an event occurred, it may be understood that the figure is approximate. A common way of justifying this trope is to make the event's first mention on an anniversary.
Often used with Time Skips and Distant Prologues or Epilogues. Not to be confused with Exty Years from Now, which is about the year in the work being directly linked to the year it was made, or Year X, which is when dates in-universe contain the letter X in place of digits. See Arc Number for when the same gap of time appears repeatedly. Super-Trope to 10,000 Years and Thousand-Year Reign.
- Bleach. The events in the Turn Back the Pendulum gaiden took place 100 years in the Soul Society's past.
- The plotline of Tenchi Muyo! has copious amounts of backstory going back for millennia; however, ALL the big important events seem to have happened a round number of years ago. To name just a few: Ryoko was imprisoned for 700 years, Ayeka and Sasami have been in cold sleep for just as long. Washu is 20,000 years old, and Kagato betrayed her 5,000 years ago.
- Dragon Ball does this in reverse; Trunks, a time traveler, comes back from the future to warn the heroes about a coming danger that will kill all the heroes in exactly 3 years.
- Beerus' naps also seem to take a round (and usually comically large) number of years.
- Issue #5 of My Little Pony: FIENDship Is Magic has Spike lampshade how every villains backstory is 1000 years ago, and his response to one that isn't? "Thank goodness."
- The use of Comic-Book Time in The DCU, and to a lesser extent the Marvel Universe, heavily involves this trope. Depending on the continuity, a superhero usually debuted either 5, 10 or 15 years ago. At one point, DC's official timeline placed the debuts of Superman and the rest of his Justice League contemporaries 12 years ago.
- Transmetropolitan uses this trope in an interesting way to keep its futuristic setting germane no matter when it is read. By hitting the calendar so hard no one recognized it anymore, people in The City simply took to referring to in-universe events in relative terms (example: a sign with an "X Years Ago" where the X is a digital display to keep it current).
- Half Past Adventure is set exactly thirty years after the finale of Adventure Time; justified because the first chapter takes place during the anniversary of said finale.
- At least one My Little Pony: Friendship Is Magic fanfic has made a joke about Celestia's many foes who she had banished a thousand years prior.
Pinkie Pie: "That must have been a very bad year for you."
- In Pokémon Reset Bloodlines, two major events of the new timeline's history took place 40 years ago. One of them being the latest Trainer-Ranger War, which is pretty much the In-Universe equivalent to the Cold War. The other is related to Professor Oak's research to pacify aggressive wild Pokémon, which was instrumental in making the world a safer and better place for trainers, to the point it was labeled as "The beginning of the Oak Era".
- In Aladdin, Genie states that he's been trapped in his lamp for 10,000 years. Who or what made the lamp 10,000 years ago is never explained.
- According to Lumiere, the curse in Beauty and the Beast has been in place for ten years. Since the curse is in its final stages, this also means the Enchantress gave the Beast roughly ten years to break it.
- El Macho in Despicable Me 2 was an active villain 20 years ago.
- How to Train Your Dragon 2 is set 5 years after How to Train Your Dragon, and 20 years after Hiccup's mother went missing.
- The Hunchback of Notre Dame contains a 20 year time skip between the prologue and the main events.
- In the short film Sintel, Word of God says that 15 years have passed since Sintel lost Scales.
- The time gap between that humans in WALLE left Earth compared to the main events is exactly 700 years; Captain McCrea celebrates by issuing everyone aboard the Axiom a septuacentennial cupcake-in-a-cup.
- Wreck-It Ralph takes place on the 30th anniversary of the release of Fix-it Felix, due to the fact Ralph was annoyed that it was now thirty years since the game's release and he'd never been appreciated for doing his job.
- Men in Black 3 was set 40 years after Agent K arrested Boris the Animal at Cape Canaveral.
- Halloween H20: Twenty Years Later is obviously set 20 years after Halloween (1978).
- One year takes place the between the accidental death in I Know What You Did Last Summer and the murderer's revenge.
- Jack Sparrow from Pirates of the Caribbean: The Curse of the Black Pearl lost his ship to Barbossa ten years ago.
- In Back to the Future, 30 years have passed since Doc Brown first got the idea to make the time machine and the time machine's completion (1955 to 1985). In the sequel, Doc Brown chooses to go thirty years into the future specifically because "it's a nice round number", but at the end of the film the time machine gets struck by lightning and goes back to 1885, which is 100 years before the start of the first film.
- In the film Tears of Steel, scientists are trying to save the world from robots by rewriting the memory of a break-up that their creator had 40 years ago.
- A man visits the museum, and an ancient sarcophagus catches his attention. He asks the museum guard: "How old is this statue?" "Five thousand years and two months." "How do you know it so precisely?" "Because I got this job two months ago, and then I was told it was five thousand years old."
- A Christmas Carol takes place exactly seven years after Jacob Marley's death. ("Marley was dead, to begin with" is the first sentence of the book.)
- In Jeramey Kraatz's The Cloak Society, the famous battle between Cloak and the Rangers happened a decade ago.
- In the Dragonriders of Pern series, threadfall lasts exactly 50 years, and intervals are exactly 200 years. Long intervals are 400 years (though technically it should be 450 to account for the period when thread should have fallen).
- Inverted in The Guns of the South, in which Afrikaaners steal a Time Machine with the intent of helping the Confederacy win The American Civil War. When Andres Rhoodie admits the truth to Robert E. Lee, the general asks why they decided to show up in 1864 (when the war was winding down) and not at a more obvious turning point like the Battle of Gettysburg. Rhoodie explains that the machine only works in increments of exactly 150 years, and since they obtained (or rather, stole) it in 2014, traveling back to 1864 was the best they could do.
- Harry Potter
- The Distant Prologue of Harry Potter and the Philosopher's Stone is set ten years prior to the main events of the book. Justified in that children are enrolled to Hogwarts at the age of 11, which is a reasonable age to start secondary education.
- In Harry Potter and the Chamber of Secrets, the Chamber is opened in 1992, 50 years after it was opened in 1942.
- In Harry Potter and the Prisoner of Azkaban, Sirius Black escapes after being in prison for 12 years
- Subverted in the Distant Finale of Harry Potter and the Deathly Hallows, which is set 19 years after the main events of the book rather than the more convenient 20.
- In The Hunger Games, Haymitch won the 50th Hunger Games, so he won 50 years after the games were first made, and 25 years before the Second Rebellion.
- Most important things in the Ranger's Apprentice series happened twenty years ago in-universe. This turns out to be justified-twenty years before the first book, Baron Morgarath rebelled against the new king Duncan, causing a civil war which led to the death of Will's father. The events of this time are chronicled in two prequel books, and a short story in Book 11.
- "The Engineer", an automaton from Skulduggery Pleasant states that it spent "almost three decades" i.e. thirty years, doing its job before getting bored.
- Tolkien's Legendarium: Bilbo Baggins in The Hobbit returns from his adventures to Hobbiton at the age of 51, and in The Lord of the Rings he leaves Hobbiton at 111. Thus, exactly 60 years pass between the two books.
- Doctor Who: In "The Eleventh Hour", the Doctor intends to leave young Amelia Pond alone for five minutes, but ends up leaving her for 12 years.
- In the Korean historical drama Emperor Wang Guhn Kyun-hwan is poisoned and the only antidote is thousand year-old ginseng root. Some is found and given to him; but it doesn't work well, because it's only 500 year old ginseng.
- Mighty Morphin' Power Rangers kicks off with Rita Repulsa's escape from the dumpster in which she was imprisoned by Zordon. In her own words, "AFTER TEN THOUSAND YEARS, I'M FREE! IT'S TIME TO CONQUER EARTH!"
- The Beatles: According to the title track of Sgt. Pepper's Lonely Hearts Club Band, "it was twenty years ago today / That Sergeant Pepper taught the band to play."
- Henry Mancini's song Le Jazz Hot begins with the lines "'Bout twenty years ago way down in New Orleans,.."
- In the song "I Was Born About 10,000 Years Ago" the narrator claims to have been born 10,000 years ago.
- The band Ten Years After, who coined their name from being formed ten years after Elvis Presley became a star.
- Ten years take place between BioShock 2 and its prologue.
- In BioShock Infinite, Booker once attempted to have a baptism (he changed his mind part way through) after the Battle of Wounded Knee, and says it "must have been 20 years ago".
- Caves of Qud is set a thousand years after a great cataclysm.
- In Final Fantasy X, it is said repeatedly in the game that Zarkanand was destroyed by Sin 1,000 years ago due to the city's reliance on machina.
- Five Nights at Freddy's 3 is set 30 years after the events of the original game.
- The plot of Ghost Trick was triggered by a meteorite killing the villain 10 years ago.
- Half-Life 2 takes place 20 years after Half-Life, but there's a good chance this was intentionally done by The G-Man.
- The Last of Us has a 20 year time skip between the fungus outbreak and the main events.
- As its title suggests, Paper Mario: The Thousand-Year Door contains a door that last opened a thousand years ago.
- Pokémon Black and White: The Pokemon Roggenrola was discovered a hundred years ago, and Elgyem was first discovered when it emerged from deep within the desert 50 years ago.
- Virtue's Last Reward takes place 45 years after the Radical-6 pandemic outbreak.
- All the dates of the Entity's memories that can be traveled to in Chrono Trigger just so happen to be nice round numbers, at least for the centuries (600, 1000, and 2300 AD; 12,000 and — wait for it — 65,000,000 BC). The "Apocalypse," though, happens in the year 1999. 1000 A.D. is at least set during a millennial celebration that indirectly sparks the events of the game, but there's no particular indication why any of the other nice round dates happen to have such important things occur on them.
- Can be attributed to Battlefield series with Battlefield 1942, later having a sequel set in the future entitled Battlefield 2142.
- Some of the, well, battlefields on which players fight are also fairly obviously linked to real battles of Wolrd War II, and so is the backstory.
- The reigning king of this trope is the Bridge at Remagen map, which is stated in its description to take place exactly two hundred years to the day after American forces crossed the same bridge as part of Operation Lumberjack in WWII.
- Some of the, well, battlefields on which players fight are also fairly obviously linked to real battles of Wolrd War II, and so is the backstory.
- Fable III takes place fifty years after Fable II, which in turn takes place 500 or 600 years after the first Fable game.
- "Nightfall": The news anchor in this adaptation reports that the astronomers have identified the eclipse as occurring every two millennium, rather than the more precisely accurate 2049 years. Probably a deliberate In-Universe invocation to play up the importance of the broadcast and keep things simple for their audience.
- In Avatar: The Last Airbender, Aang was frozen in an iceberg for 100 years.
- In The Legend of Korra, it has been 10,000 years since the time of the first Avatar, Avatar Wan, and the Harmonic Convergence, when the spirits Vaatu and Raava must do battle, occurs every 10,000 years.
- In Xiaolin Showdown, pretty much every major catastrophe has happened before, usually 1,000 or 1,500 years ago. Eventually, the show started lampshading the heck out of it.
Raimundo: imitating Master Fung "Every thousand years..." Gotta be bad news.Master Fung: ...every thousand years!
Kimiko: Why a thousand?
Master Fung: Actually, it's 962, but a thousand sounds more ominous.
- Pretty much everything major in My Little Pony: Friendship Is Magic happened a thousand years ago: Luna's banishment, Discord, King Sombra and Tirek. The first one is the only one to literally be a thousand years ago — since Luna was involved in everything else, those things logically happened some time prior.
- The Simpsons:
- Episode "Homer vs The Eighteenth Amendment" reveals that alcohol was banned in Springfield 200 years ago, but had apparently never been enforced, to the point where it was forgotten about. It isn't until the end of the episode that it is revealed that the real reason it was forgotten was because the ban was removed 199 years ago.
- Homer Simpson in "The Simpsons: Treehouse of Horror X": "Okay Marge, you hide in the abandoned amusement park! Lisa, the pet cemetery! Bart, spooky roller disco! And I'll go skinny dipping in that lake where the sexy teens were killed one hundred years ago tonight!"
- Parodied in one episode where Bart watches a movie with two teens making out in an abandoned building. When the girl questions if everything is all right, the boy assures her that nothing has happened there "in ten years. Ten years tonight."
- Lampshaded/parodied in the Futurama TV movie Bender's Game.
Nibbler: It all began 36 years ago... (glances down at his watch) ...now!
- The French animated film Gandahar directed by Rene Laloux has The Hero Silvain placed in a stasis tube for exactly one thousand years, in case the great brain Metamorphe goes rogue as the prophecy predicts. The tribe of deformed mutants quotes that prophecy to Silvain: "In a thousand years, we were doomed. A thousand years ago, we will be saved." This extends to their speech pattern, using "was/will be" as a regular conjugate of "be." The Timey-Wimey Ball bounces all over this one.
- The first words from The Pastmaster during the first episode of Hanna-Barbera's SWAT Kats are: "I am the Pastmaster, and I have been imprisoned [points to his sarcophagus] for 800 years." Either the grave robbers that dug him up timed it nicely, or the undead gnome is using rounded figures.
- In The Smurfs episode "Never Smurf Off Till Tomorrow", six Smurfs end up in a volcano. Handy Smurf examines the walls and says the volcano must erupt about once in every 500 years. The ground suddenly shakes, prompting this exchange:
Brainy: When did the last eruption take place?Handy: (thinks, writes some numbers onto the ground) Well, If My Calculations Are Correct, it's been exactly 500 years ago... to the day!