Year Zero is an event used to reckon time within a particular continuity or universe, especially by fandom.
While seemingly as arbitrary as any hypothetical calendar is, it avoids being too specific while still setting a hard date on important things. This is useful if a writer wants to avoid accidentally invoking Period Piece baggage by using certain dates, but also an easy way to set events by the narrative rather than the implication that time is passing in a strictly consistent way. In some works Year Zero is generally the "hardest", and sometimes only, date relevant to actual characters. Most of the story takes place in the conveniently nebulous period after it. Because lying between two hard dates could be restrictive given enough time, events are reckoned as before or after Year Zero instead.
Occasionally writers slip and throw in an actual date, especially in early versions or installments of works before the writers became concerned with looking dated. Fans tend to jump on these while working out timelines.
For obvious reasons, Year Zero is often invoked for authors who use sliding timelines. For example, the formation of the Fantastic Four was occasionally used as the Year Zero of Marvel Comics continuity, with 'present-day' stories occurring about 10 years after this. This does not always work perfectly.
See also Alternative Calendar, for ways of reckoning time in-universe. Events that prompt these alternatives have Hit So Hard The Calendar Felt It.
Compare Episode 0: The Beginning.
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Gundam has a different Year Zero for each of its Alternate Universes, though they were invented by the creators rather than the fans. Interestingly, each series takes place decades after its timeline's Year Zero, and only rarely is the Year Zero explicitly tied to a specific event.
U.C. 0001, the first year in the original Mobile Suit Gundam's Universal Century, is the year in which migration to space begins, and as revealed in Mobile Suit Gundam Unicorn (the first UC series to make reference to that year) is also tied to the actions of a young Syam Vist.
In Gundam SEED, the Year Zero appears to be a Year 1: The Cosmic Era calendar is adopted in roughly 9 CE, and is retroactively dated to begin when nuclear weapons were used in Kashmir nearly a decade before. Three new superpowers and a number of new smaller powers also emerge from the radically changed national order. Following other Gundam timelines, the UN also announces a new Space Exploration and Colonization program. Much like other timelines, it's not mentioned when the exact date AD became CE, other than that it was sometime in the mid- or late-twenty first century.
Fans tend to attempt to create the regular version of this with at least the Universal Century timeline, since there are a handful of specific dates given that are tied to a day of the week. This can be used to narrow down 2045 as the earliest possible year for the switch to the new calendar.
The After Colony calendar in Gundam Wing on the other hand, is implied begin from the launch of Skylab in the 1970s. Though it's never mentioned exactly when AC replaced AD.
Lyrical Nanoha measures time based on the new Mid-Childa calendar, which according to the Fate's As You Know talk in episode 14 of Magical Girl Lyrical Nanoha Strikers, began during the period when physical-based weapons were banned and Magitek became the standard. This was the time when the TSAB had fully established themselves, building their main branch in Dimensional Space and their land-based HQ in Mid-Childa.
When Kagome travels to the past in InuYasha, she plops down into Japan's Sengoku Jidai, a century of continuous civil war and bloodshed. Because no exact dates are mentioned it would be anyone's guess as to what year it is when she goes back in time. The main clue the fandom has used to try to put a date to the era comes from one of the very first episodes, where a young samurai mentions Oda Nobunaga, but considers him to be unimportant and an idiotnote Nobunaga eventually brutally conquered much of Japan and his successors finished the unification and established the government that ran the country for the next 250 years. Since there is a rather short window of time where Nobunaga was known, but not as a terrifying, ruthless, slaughter-happy Bad Ass, fandom uses this to get a rough idea of when the story is supposed to be taking place.
The Year Zero event in One Piece is generally portrayed as the founding of the World Government, which occurred approximately 800 years ago. This event is the foundation of the One Piece world as it is now and the end of the Blank Century, a period which has very few surviving historical records and is illegal to study.
Another Year Zero used unofficially in-story is the death of Gold Roger, twenty-four years prior, which started the "Great Pirate Era." Many characters refer to the "current era" and how it started with Roger's death. Some also remark that Whitebeard's death is yet another Year Zero, creating a new era.
Time in Star Wars is generally measured in relationship to A New Hope. This was eventually given a nod with the introduction of an in-universe calendar established by the New Republic based on that same date, specifically, the Battle of Yavin.
Ironically, many fans object to this Fandom Nod, considering the Battle of Endor in Return of the Jedi to be a more logical date to base an in-universe calendar around (since the events of ROTJ had far more in-universe significance).
The Star Wars BBY/ABY (Before/After Battle of Yavin) calendar differs from most real-life examples in that 1 BBY and 1 ABY are the same year. The transition between BBY and ABY happens at the exact moment that the Death Star was destroyed.
The use of the Battle of Yavin as a starting point for a calendar makes more sense if you remember the Opening Crawl for the first Star Wars film, which mentioned that just before the movie begins, the Rebels had won their first stand-up battle against the Empire. The scene where one Imperial officer is very nearly Board to Death for having a disturbing lack of faith has the Imperial officers discussing what to do about this recent development. So the Battle of Yavin presumably works because it was A) close to the beginning of the Rebellion proper, and B) a much more significant victory than whatever happened before the film started rolling.
Germania anno zero (English trans. Germany, Year Zero) was a 1948 Italian film set in post-WWII Berlin. Protagonist Edmund Kohler (age twelve) struggles to survive in a chaotic world: little rebuilding has been accomplished, food and necessary supplies are rationed & shortages are common. Edmund escapes the tense, claustrophobic apartment—in which he resides, along with remnants of five families—to the streets, hoping to scavenge useful items or information. Things get worse.
In the early days of Harry Potter, fans had to measure time from Harry's first year at Hogwarts. Thankfully for timeline-makers, Chamber of Secretsthrew in a line (specifically, Nick's Deathday date) that placed the first book in the 1991-92 school year. Deathly Hallows established the same time frame through the dates on James and Lily Potter's graves. Despite several uses of post-1998 technology and London landmarks, the films (filmed from 2000-2010) also have gravestones dating the films' "present day" to this period.
Though there were always some details here and there that showed in what year the Sherlock Holmes stories were set, there was always the question on how old was Holmes. Whether he was in his 20's, 30's or 40's during Study In Scarlet (his first appeareance). It wasn't until His Last Bow (which was supposed to be the Grand Finale) that he decided to give Holmes an age of 60 during 1914, meaning that he was born in 1854 and was 27 years old during Study in Scarlet (set in 1881)
J.R.R. Tolkien's timeline in both The Hobbit and The Lord of the Rings was meticulously kept. Frequent dates were given on the in-universe calendar (in the case of The Lord of the Rings both in Shire Reckoning and the larger calendar of Middle-Earth). Additionally, each culture kept time based on their own internal reckoning, with years based on significant events: The Hobbits followed Shire Reckoning, with Year Zero on the founding of the Shire, while Gondor and Rohan had their own calendars as well. Middle-earth on the whole broke timekeeping down into different Ages whose first year was decided by specific events. Tolkien paid so much attention to his calendar system that he even rewrote the entire chapter of Faramir and Frodo overlooking the waterfall at Henneth Annûn because he realized that the phases of the moon he used didn't match his previously established timeline!
The timeline for the Silmarilion, however, is somewhat more confused and murky, possibly owing to his death before completing it. Exactly how the Valian Years, the Years of the Trees and the Years of the Sun in the First Age connect is left unclear.
In A Song of Ice and Fire, Year Zero in the Seven Kingdoms is dated from the crowning of King Aegon I the Conqueror, founder of the Targaryen dynasty. The main series starts 298 years later.
At the end of Anathem, the main character notes that the current day is "Day Zero, Year Zero". Given that their discovery of, and contact with, other universes has changed society a lot, this is to be expected.
In the Honor Harrington universe as a whole, Year Zero is the Diaspora, when humanity left earth for the stars. All years are usually referred to as Standard Years Ante or Pre-Diaspora. Planets also have their own local calendars, using the initial landing of colonists as their respective Years Zero.
Also from David Weber is Safehold. As far as Safeholdians know, Year Zero is the day of Creation itself. Though following the Archangel Shan-wei's rebellion, a new calendar was established using the end of that war as the Year Zero.
In Frank Herbert's Dune, the imperial calender starts at the creation of the Spacing Guild, which holds a monopoly over interstellar transportation. It is never explained why this event was chosen as the reference over, say, the coronationof the first emperor or the discovery of the spice.
In Robert Reid's Year Zero, citizens of the universe revere artistic endeavors, none more so than music. Turns out that humans make better music than any civilization in the history of the universe. The discovery of Earth's music (sans North Korea) was so monumental that they reset the universal calendar based on the date of discovery of the theme song to Welcome Back, Kotter.
Live Action TV
Andromeda measures years in Commonwealth Years, which starts with the beginning of the Vedran Empire, making time in the series very non-specific. However, extended material gives the Commonwealth date for when Friedrich Nietzsche published Thus Spake Zarathustra, a definite date in our time system, so assuming they use the same years as we do (years based on the cycle of Tarn Vedra rather than Earth, though the difference can't be too much), it creates a good guess of the timeline of the series.
The Battlestar Wiki Timeline measures time from the Cylon holocaust depicted in the Battlestar Galactica miniseries. The in-universe calendar is only referred to once, in the season 3 episode "Hero", where readable text in a printed copy of Adama's dossier indicates that the 'present day' of that episode is the Colonial year 21356, somewhere around the year 148000 B.C.
The current year is never mentioned in any season of 24, but the amount of time between days (16 months between Day 1 and 2, three years between Day 2 and 3, etc) is always established and can be used to determine how many years it has been since Day 1. In addition, fans have used various clues to formulate the following timeline:
Day 1: Tuesday, March 2nd, 2004
Day 2: A Saturday in September 2005
Day 3: September 2008
Day 4: March 2010
Day 5: September 2011
Day 6: May 2013
Day 7: February 2017
However the real world events that surround the first season would make this timeline slightly incorrect. The first season is meant to take place around one to two years after Jack's mission in Belgrade. The mission took place when Slobodan Milosevic was still in power which would place the mission in either 1999 or 2000.
The season 2 finale of LOST established that Oceanic 815 crashed and started the whole thing on September 22, 2004, the same day that The Pilot, Part 1 episode was first aired and, you know, started the whole thing. Since then, fans were able to give all events of Seasons 1-4's present day an exact date based on the clues within the series. However, a Time Skip appeared in season 3 and events from flashforwards in that finale and season 4, and any of seasons 5-6 can only be put down to the year they happen in, with no precise dating except in relation to each other (with the exception of one season 4 flashforward, and time skips in season 5 that overlap with events from the early seasons).
The album Year Zero by Nine Inch Nails is a Concept Album and associated ARG in which 2022 (15 years after the album's release) has been declared "Year Zero" by a corrupt, far-right U.S. government.
Final Fantasy VII has the Shinra Company setting the year zero of its new era at the end of the Wutai War, six years before the start of the game. While not very important in the original game, games set in the prequel portion of the Compilation often make note of the date.
The Elder Scrolls universe has several Year Zeros, at the beginning of every new 'era'. It's common practice to put the era before the year when announcing a date, for example "Third Era, Year 433". The first four games take place in the 5th century of the third era (roughly - Arena covers the last year of the 4th century), and Skyrim takes place in the third century of the fourth era.
The calendar in the Spanish webcomic 5 Elementos is based on a date called "Day Zero", when there was an all-out war that made the whole country collapse (curiously referred to by no other name than the "Day Zero war"). However, no date has been given so far in the comic, we only know the base of the calendar. (In fact, the author played with this in a chapter, trying to get fans off track in the timeline before revealing that Rubéola wasn't Matarratas's real sister, when Matarratas's father had been stated to be 3 in the Day Zero.) Current fan speculation has the first chapter at the year 32.
Every so often, this happens with revolutionary governments who want to stress a complete break with The Way Things Have Been Done Heretofore. The Ur Example of this may be from the French Revolution, particularly after the ascendancy of the Jacobins, when a new — Metric — calendar was introduced, restarting the year numbering at Year 1. Similarly, the "Juche" calendar of North Korea has been recalibrated from Kim Il-Sung's birth year.
A literal example of this trope comes from Cambodia, where the Khmer Rouge regime under Pol Pot declared 1975, the year that that Khmer Rouge insurgents were able to seize control of the Cambodian state, to be Year Zero of a new calendar.
This was averted by the Anno Domini dating system used throughout the world. There is no year zero, and it is one of the calendar's base assumptions. Things that happened before the birth of Christ (the starting point of the calendar) are calculated in negative years Before Christ, counting down not to Year Zero, but AD 1: that is, there is a direct skip from 1 BC straight to AD 1! (Due to errors in calculations, the actual birth of Christ may lie anywhere from 6 BC to AD 6.)
Played straight on both the astronomical calendar and the ISO 8601 specification. Both include a year zero, as the lack of this screws up the math on BC years. Thus, 1 BC is 0, 2 BC is -1 and so on.
The time when Nazi Germany was defeated by the Allies and reconstruction efforts began was known as "Die Stunde Dull" or "Das Jahr Null" (Zero Hour / Year Zero) to German citizens. It combined a feeling of defeatism, utter shock, delusion, and hope for the future.