Hit So Hard the Calendar Felt It

It's The End of the World as We Know It, and The End of the Beginning as well. Either the few remnants of sentient life (we're talking either Class 1 or 2 here) are rebuilding, or some Reality Alterating Event has taken place which reshapes their entire conception of History. In any case, they see fit to reset the calendar and set civilization's New Beginning as Year Zero (or year one).

Can also be revealed to have happened in retrospect as a World Building trope - the event after which a society dates its calendar tells you something about their culture.

Not to be confused with people being "punched into next week".

Anime and Manga

 * The Gundam franchise loves using alternate calendar systems, but the only one that really matches this trope is Gundam X, which is set in After War 0015. Some fan theories suggest that Gundam Wing's After Colony calendar started with the launch of Skylab, but there isn't any official proof that this is true or not. Special mention to Turn a Gundam, which implicitly has several dozen calendar changes in its backstory, to the point where this trope is the only explanation.
 * The setting of Battle Angel Alita: Last Order marks time according to the Era Sputnik, counting from the launch of the Sputnik 1 in 1957.
 * Nobody remembers what year it is in The Big O because everyone was hit with Laser-Guided Amnesia forty years ago. As a result, people tend to refer to the date by referencing the loss of memory, "X years ago".
 * One of the proofs that the Holy Britannian Empire is the most dominant force on Earth in Code Geass universe, the calendar doesn't follow the real world AD/CE - instead, we have A.T.B, for "Ascension to Throne, Britannia". This roughly coincides with about -60 BC, when the in-universe history diverges (the reason being that Julius Caesar never conquered Britain; the Anglo-Saxons elected a "super leader" to fight him instead and won, amongst all the historical variance in that universe which included a failed American Revolution and a change of seat of power from mainland Britain to American Colonies).
 * In Lyrical Nanoha, the new Mid-Childan calendar begins after the Ancient Belka War, which ended when the dominant Ancient Belka Empire destroyed itself and rendered their planet uninhabitable due to a combination of infighting and the Lensman Arms Race, leading to the banning of mass-based weapons on all dimensions.
 * At some time, possibly after the War with Them, the So Ra No Wo To calendary was changed to A.P.

Comic Books

 * Transmetropolitan takes place at some unspecified year in the future. No one ever refers to a specific year; it's always in reference to other events. At one point, we're told that a Revival truly lost it when she asked what year it was, and "they told her." It's not stated outright, but what they probably said was "we have no idea."
 * The "Ultimatized" Marvel 2099 seen in Timestorm 2009-2099 includes a brief scene set some years earlier with a holographic teacher modeled on Reed Richards explaining that it probably isn't really 2085 because they started counting again from the last date before the disaster.
 * Hypernaturals by Dan Abnett and Andy Lanning is set in the year 100 AQ (Anno Quantinuum), a century after The Singularity.

Fan Works
"Gondorians: *change calendar*"
 * In With Strings Attached, the planet C'hou underwent some kind of big shakeup about five hundred years previously. Among other things, it seems to have removed all concept of calendars and clocks from Baravada.
 * The propensity for this in The Silmarillion is referenced in the abridgement The Entire Silmarillion of J. R. R. Tolkien in One Thousand Words:

Film

 * 2012
 * In the '50s movie When Worlds Collide, a rogue planet passes by Earth and destroys it due to tidal effects, but the passing planet itself is inhabitable. In the end, a few people land on it to re populate it, and an ominous Bible-like title card says this is Year One.
 * Titan A.E.: The movie begins A.D. 3028 and ends 15 years later, in the year 15 A.E. (After Earth).
 * In Star Wars, time is divided into BBY and ABY, before and after the Battle of Yavin (in order to distinguish prequels from the original trilogy).
 * The Expanded Universe gives at least two more times this happens prior to Yavin.
 * In Land of the Blind, the calendar is reset with the year of the revolution as Year Zero. The film begins in Year Minus Five.

Tabletop RPG

 * First Edition Paranoia: Dates were sometimes in the form of "Year 240 of the Computer"; i.e., 240 years since the Big Whoops (destruction of world civilization) and the takeover of Alpha Complex by the Computer.
 * It became even weirder in later editions where it was implied that, due to some sort of glitch, EVERY year is "Year 240 of the Computer". Although anyone who pointed this out would be implying that the Computer is defective, which is, of course, treason.
 * Years in Dragonlance are numbered from the Cataclysm.
 * One of Forgotten Realms' several calendars is "Present Reckoning", started on Time of Troubles. It never caught on, though. This fact is mentioned only once in the 3rd Edition books and everything else from then on is only the standard Dales Reckoning.
 * Exalted had its timeline initially measured in years pertaining to the Age of Man, when the Primordials were overthrown and the Solar Deliberative was established. When the Solar Deliberative fell during the Usurpation, the Shogunate fiddled with an alternate calendar. At least until the Great Contagion fucked with that and the Scarlet Empress ended up unifying various daimyos into the Realm, which is why the calendar currently uses RY (Realm Year) to designate how many years it's been since she took the throne.
 * The calendar system basically says, "It has been X years since the last world-shaking cataclysm."
 * The German RPG Das schwarze Auge (Realms of Arcania) had the main realm count the years by the current Emperor. By now reckoning has been changed (back) to the 'BF - Fall of Bosparan' reckoning, counting from the fall of the former empire. There are also numerous other calendars around in that world, counting from the first landing of gildenland settlers, the independence of countries, the threat of a catastrophe by a messenger of the gods to a city, and so on.
 * Eclipse Phase uses AF (After Fall), mostly as a way to avert Exty Years From Now. ("The Fall" being when a bunch of crazy AIs nearly wiped humanity out.)
 * On Dominaria, the default world of Magic: The Gathering, most nations use the Argivian Reckoning calendar, which sets Year 0 at the year the Brothers, Urza and Mishra, were born (Urza on the first day of the year, Mishra on the last). They were the generals for the opposing forces in the Brothers' War, which destroyed multiple kingdoms, wrecked the continent of Argoth, and ushered in the Ice Age.
 * Rifts's Post-Apocalypse calendar came about when enough people realized that they had enough time and energy left over from trying to survive after The End of the World as We Know It (two or three centuries after it actually happened) to care about trivial matters like what year it was.
 * Ironically, averted in Warhammer 40,000, which still uses our current calendar without even a token epoch shift. Though heavily modified (it's the only well-known calendar that takes into account the uncertainty of timekeeping in the multistellar empire), it's still the good old Gregorian calendar based on the Earth year and the birth of Christ. Probably they still use it because it's one of the few things they've managed to keep from the Dark Age of Technology, Humanity's lost Golden Age.

Literature

 * Edgar Pangborn's Davy and related stories take place in North America centuries after a 'limited' nuclear and biological war. Some years after the war a prophet named Abraham arose, and the church founded on his martyrdom established a new calendar from that date, with The Year of Abraham replacing Anno Domini.
 * The Dune books did something analogous, resetting the calendar when the Spacing Guild established its monopoly (which was about 108 years after what might have been regarded as the apocalypse — the end of the Butlerian Jihad).
 * It is a little strange that, according to the prequel novels, the Imperium was established in 88 BG (Before Guild) at the end of the Battle of Corrin (the Jihad itself ended 20 years prior), which resulted in the destruction of the last of the Thinking Machines . After all, would not the re-establishment of a single empire be the moment to reset the calendar?
 * Transportation is one of the most time-obsessed industries there is. It's logical that when one company takes over all inter-stellar transportation, their timekeeping regime (optimized for efficiency and accuracy) would become a standard.
 * Little is known about the Old Imperium (pre-Thinking Machines), although it is possible they still used the Gregorian calendar. No Gregorian date is ever given, but it can be calculated from the information in the novels that 0 AG = 11,401 AD (or CE), plus or minus 400 years. This means that Dune takes place in 21,267 AD (10,191 AG).
 * In Drowtales, the current story takes place in the year 1098 of the Moonless Age. Not because the moon was destroyed 1100 years ago, but because the elves went into exile underground where they couldn't see the moon.
 * In The Lord of the Rings, the story begins in the year T.A. 3018, where T.A. stands for "third age" (meaning they'd already reset the calendar at least twice), and by the end of the story, a fourth age is declared.
 * Tolkien's verse was hit so hard the calendar felt it thrice. The first time when Morgoth was defeated and Beleriand sunk. The second time when Numenor sunk, and refugees from it founded Arnor and Gondor, and defeated Sauron. The third time, you know.
 * It was hit more times than that: The Silmarillion mentions a number of events that occur before the beginning of recorded time, and later mentions that Morgoth was bound for 'three ages of the world' - and all of this before the First Age itself actually begins.
 * Specifically, the Age of Creation, the Age of the Lamps, the 2 Ages of the Trees (Pre-and-Post-Elf awakening), and the Four Ages of the Sun (1st ends with the defeat of Morogoth, 2nd ends with the defeat of Sauron at the Last Alliance, the 3rd ends with the final defeat of Sauron).
 * The calendar is actually more complicated than that. Tolkien himself NEVER described a "First Age of the Sun." The First Age is considered to have begun with the awakening of the Elves partway through the Years of the Trees (long before the first appearance of the sun). The first 5000-odd (to even as long as 65,000 depending on how exactly the years are determined, which is unclear) years of the First Age were reckoned as the Valian Years. The sun itself didn't appear until the last 590 years of the First Age and were dated as the Years of the Sun. However you could rightfully date events during that last half-millennium of the First Age as either Y.S. (years after the first rising of the sun when the Noldor returned from Valinor) OR by its date in Valian Years (although the conversion is unclear). Let's also not forget the Shire Reckoning, which began about halfway through the Third Age and dates to the founding of the Shire (and didn't reset with the onset of the Fourth Age) and that there's mention of similar calendars among some other peoples (there may have been mention of the Rohirrim following a calendar based on the establishment of their kingdom and of the Men of Númenor doing the same, but I can't remember off-hand).
 * The Legend of Rah and the Muggles has the calendar being reset by a nuclear war. The problem is that they talk about the "year of the purple haze" [nuclear fallout]. Every single year in living memory has been the year of the purple haze!
 * In The Stand, Randall Flagg tells the date as "this thirtieth day of September, the year nineteen hundred and ninety, now known as The Year One, year of the plague."
 * The Turner Diaries is set in 1991 and 1993. During the epilogue, it is explained that around the year 2000, they reset the year to coincide with the annihilation of all minorities.
 * In The Bible, the events of Passover shifts the calendar to starting the year at that month.
 * In The Wheel of Time, the historical records show several calendars being used. This has happened 3 times in recorded history: The end of the 2nd Age and the Breaking of the World, the Trolloc Wars, and the Hundred Years War. The story occurs at the end of the 3rd Age, so the calendar is likely to reset again.
 * In Charles Stross's novel Scratch Monkey, by the time of the book's setting (several thousand years from now), there have been so many rulers declaring new eras and new calendars that "Year Zero Man" has become the standard term for a totalitarian dictator.
 * In Tamora Pierce's Tortall Universe, the numbers of the years are followed by "H.E.", short for "Human Era"- the time after the Immortals were banished to the Realms of the Gods.
 * In the Codex Alera series, it is eventually revealed that the calendar is reset by the events of the series.
 * The hyperconsumerist society in Brave New World worships Henry Ford, and dates its calendar from the release date of the Model T.
 * In The Belgariad and its sequel, The Mallorean, the calendar used dates the year 0 to be when the evil god Torak cracked the world. This particular calendar is referred to as the Alorn calendar, implying that other nations may have different systems.
 * The Tolnedrans at least are implied to date by dynasty.
 * The Dals measured time in "Ages", which only ended when something momentous to their grand mission occurred. The cracking of the world corresponded to the end of their First Age, and the books take place in their Fifth Age.
 * The Mortal Engines have all dates measured from TE - Traction Era, i.e. the time from which cities started moving around gobbling each other up.
 * In the Honorverse, the Galaxy at large uses the Ante/Post-Diaspora calendar, which shifts the epoch to 2103 CE at the launch of the first colony ship, the Prometheus, but is otherwise a standard Gregorian calendar, measuring in T-years (Terra years) and T-centuries and so on.
 * Many planets also have their local calendars, usually dating from their founding (Manticore, for example, has the AL system, based on the arrival of the Jason in the Manticore Binary System and keyed to Manticore's orbit of 1.7333 Earth years to the year, in some official documents), but everybody respects the Diaspora calendar...except backward places like Grayson, which still uses the pure Gregorian calendar (which is particularly annoying because "AD" means "Ante Diaspora" to everybody else, so "4000 AD," which is around where the series takes place, means roughly 2000 BCE to non-Graysons...).
 * Riddley Walker uses the O.C. system, which stands for "Our Count."
 * Implied as Tally and Zane look at Rusty graffiti in the second book of the Uglies trilogy.
 * The far-future bits of the Enderverse (Speaker for the Dead, Xenocide, Children of the Mind, and some short stories) dates events from the foundation of Starways Congress, the interstellar pan-human government established 1180 years after Andrew "Ender" Wiggin defeated the Buggers (an event implied to occur sometime between the 24th and 27th centuries AD). Speaker for the Dead starts in the late 20th century SC.
 * H. Beam Piper's Terro-Human Future History used Atomic Era dating, with 1942 AD, the year of the first sustained nuclear chain reaction, as Year 0.
 * So does Larry Niven's stories of Svetz the Time Traveler.
 * Ian Stewart's Hoard of Mathematical Treasures includes a "Future History of Mathematics", which parodies the genuine "History of Mathematics" earlier. Towards the end (after 11828 AD) we get "0: Reformation of the calendar", but what caused this is unknown. (Although it does seem to occur shortly after human civilisation rebuilds itself from machine rule.)

Live Action TV

 * Star Trek reckons dates in Stardates, perhaps in the aftermath of the Eugenics Wars or one of the other watershed events in the 21st century.
 * Actually, Stardates are supposed to a unified system in a Federation where planets have different days and years, and where starships travel at relativistic speeds and can be affected by time dilation.
 * There is a bit of Canon Discontinuity because of the differences in uses between the events of the original series and the time of The Next Generation By the time of the latter, the Stardate timeline became more firmly established, with Stardate 41000 being the beginning of Earth year 2364 and advancing steadily so that 1000 passed by the beginning of the next Earth year. All events within the continuity of The Next Generation (including Deep Space Nine, Voyager, and the three TNG movies) obey this timeline.
 * The writer's bible for TNG says that the "4" in "41000" signifies the 24th century, and the TOS stardates really started with 3, but it was just omitted in practice.
 * The 2009 film throws even more Canon Discontinuity into the mix by changing the Stardate system yet again. The reboot abandons the system established in TNG for a system where the Stardate always matches the year on Earth. Even events that took place on the original timeline use the new system. As a result the future events involving Spock and Nero take place on Stardate 2387.XX (the system established by the Abrams film) instead of 64XXX.XX (the system established by TNG and it's spinoffs)
 * Blakes Seven takes place in the third century of the second calendar. It's never quite clear why they stopped using the first calendar, though.

Video Games

 * Invoked in Grand Theft Auto: San Andreas. When a guy called The Truth gets something CJ stole from a government base, he says, "They shall call this Year 0."
 * The Elder Scrolls universe has this happen at least three times.
 * Both Xenogears and Xenosaga share the same in-universe calendar "Transcend Christ." It is identical to the Gregorian calendar, but it reset the year numbers so that AD 2510 is year one. A short time after the new calendar is chosen, humanity leaves Earth.
 * In the Star Ocean games, the calendar was reset in 2091 so that 2087, the year of the invention of the first FTL engine, was year one. Dates are marked as 'SD' - spacedate.
 * The Chrono Quake in the Galaxy Angel games.
 * The Dragon Age Series measures time against the establishment of the Andrastian Chantry and the crowning of the first Divine, with the current "age" (a period of one hundred years) used as an additional reference. Thus, something that occurred three years before that would have taken place in -3 Ancient (Age). Origins takes place in 9:30 Dragon (the 30th year of the Ninth Age, or "Dragon Age," 829 years after the crowning of the first Divine).

Webcomics

 * The four "breakings" in The Dragon Doctors each started a new calendar. The current year is "625 4B".
 * The The Order of the Stick is set in the year 1183 (now 1184). The calendar appears to date from the creation of the world. (Or its recreation, as the original world was destroyed.)
 * In Jack, the calender got hit so hard it restarted back to the Biblical Genesis.
 * Homestuck: In the post-Scratch universe, is marked "0 Post Condescension", and years are counted from there.

Web Original

 * Orion's Arm uses the Lunar calendar based on the landing of Apollo 11 in Tranquility basin (years denoted AT, After Tranquility), after the Nanodisaster and the evacuation of Old Earth.
 * The two sequels to The Intercontinental Union of Disgusting Characters use years before and after the IUDC as their only system of datekeeping.
 * The world of Shadowside marks the start of Evo's Dawn in A.E. 2007 with "P.E.D. 0". (Post-Evo's Dawn)

Real Life

 * The ancient Romans dated their calendar from the foundation of the city of Rome -- ab urbe condita or A.V.C.. Their original calendar divided the year into ten months: Martius (Mars(=Ares)'s (Mars being the god of war and farming, activities beginning in the spring)), Aprilis (Opening, i.e. of buds), Maius (Maia (a fertility goddess)'s), Iunius (Juno(=Hera)'s), Quintilis (Fifth), Sextilis (Sixth), September (Seventh), October (Eighth), November (Ninth), and December (Tenth). When the ten month calendar proved unworkable, two new months were added at the end, Ianuarius (Opening or Janus's) and Februarius (Purification, referring to a winter festival). Julius Caesar, when reforming the calendar (and naming the whole system after himself) renamed Quintilis, the month in which he had been born, Iulius after himself, and Augustus renamed the following month Augustus after himself to commemorate the Battle of Actium. It was quite some time later that the beginning of the year was shifted around: it was changed in the 2nd century B.C. in order to allow the newly-elected consuls to be ready for battle by the start of the campaigning season (spring).
 * Speaking of the consuls, the other major Roman dating method involved referring to a particular year as "[the year] X and Y being consul," which, naturally, was the year when the two individuals in question were consul. During one of Julius Caesar's consulates, his co-consul was such a pushover that people took to referring to that period as "Julius and Caesar being consul."
 * A 6th century monk called Dionysius Exiguus ("Little Dennis") is believed to have been the first to have dated the calendar from the birth of Jesus of Nazareth; his calendar was popularized by its use in Bede's writings, leading to the B.C. (before Christ—in some older documents A.C. for Ante Christum is used) / A.D (Anno Domini, "in the Year of Our Lord) system most commonly used today. Dionysius's calculations, however, were criticized as inaccurate as early as the 12th century, and were not universally adopted even in Christendom. In the 19th century, the use of "B.C.E." (before common era) / C.E. (common era) was proposed instead. Adoption has been mixed.
 * The shift from the Julian to the Gregorian calendars took place among Roman Catholics in the 16th century, but was not commonly adopted among non-Catholics until about the 18th, leading to a difference of some eleven days. This is why some dates like George Washington's birthday will appear as February 11 (O.S. (Old Style)) and February 22 ((N.S.) New Style) and why Miguel de Cervantes and William Shakespeare died on the same date (April 23, 1616), eleven days apart.
 * China and Korea changed calendars every time there was a new emperor. This, as well as numerous gaps in written records, is why China's history is so maddeningly vague sometimes.
 * Japan, still having an emperor, does it as well. 2010, for example, was Heisei 22 and in May 2019 it reset to Reiwa 1. Unofficially most people just memorize the important dates that will be on government forms (such as their birth, marriage and birth of their children) plus the current year and struggle with conversion beyond that.
 * This is a concept called regnal years, which has been used by many monarchies throughout history, including in the West, and is not unique to Japan or China. However, references to regnal years in the West are typically only seen in limited contexts with direct links to the monarchy; the most common one is probably references to regnal years in the Acts of the UK Parliament until 1963 (when they were replaced with Gregorian years) and Canadian Parliament to this day.
 * We should note that the common use of A.D. is, from a Christian point of view, a prime example of regnal-year numbering: Christ is the King of Kings, and since His first arrival on Earth, he has reigned for [current Gregorian year] years. The C.E. thing builds off this by saying, more or less, "We don't accept your religious beliefs, but damn does having one calendar for everyone make things so much simpler, and damn do you Europeans have one of the best calendars around."
 * There was a movement to redefine the calendar as starting from the birth date of Sir Isaac Newton. Obviously, it never came to pass.
 * Newton himself would probably have thoroughly disapproved, as he was a devout, though highly eccentric, Christian.
 * That might be why this movement only started years after Newton died.
 * After the French Revolution, not only did they restart the calendar, they made up an entire new month/week/day system. Didn't catch on. This was actually a little odd, as nearly every other unit the French revolutionaries came up with did catch on (they are collectively now known as the metric system). Probably metric time didn't catch on because it was by far the most complex system of units, as well as being by far the most tied to revolutionary France. It also cut down rest days from one day in seven to one day in ten. Predictably, this wasn't popular with the mob.
 * North Korea has the Juche calendar based on the birth year of Kim Il Sung. It did not replace (yet) the common world calendar but most North Korean publications include both dates. For instance, something published in 2010 will be dated "2010 / Juche 98". Also, yes: The 100 year mark is 2012.
 * Taiwan has a similar system, which is based on 1911 (the establishment of the Republic of China) being Year 0. 2010 would be "the 99th year of the republic".
 * When Pol Pot took control of Cambodia, he abolished the old calendar and declared the first year of his dictatorship to be Year Zero.
 * In Fascist Italy, Roman numerals were used to denote the number of years since the establishment of the government in 1922.
 * Veeery close to the definition of this trope is the Islamic calendar ("Hijri"), which started when Muhammad and his followers emigrated from Mecca to Madina. The enemies of the Meccan Muslims were about to assassinate Muhammad.
 * Midnight on January 1, 1970, was chosen as "Second 0" to define the UNIX epoch. It was picked arbitrarily and not expected to be around for long, especially since we are going to run out of digits around 2038.
 * UNIX time is still in use, although now 64-bit timestamps are becoming more common (especially on 64-bit operating systems), which avoids this problem (which only occurs if signed 32-bit timestamps are used).