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In Speculative Fiction, it's common to use a different calendar than the real world. This makes it clear to the reader that the story takes place either in another world, or in a version of our world so far in the future that time isn't even counted the same way. This also elegantly sidesteps the problems of Exty Years from Now.

In fantasy, a popular version of this is to measure time in "moons" instead of months. In some cases, the author will actually have twelve different names of the form "______ Moon" to replace the twelve months of the Gregorian calendar. Nonetheless, there are actually somewhat more than twelve lunar months in an Earth year. Real lunisolar calendars solve that problem by adding a leap month to certain years; some purely lunar calendars (like the Islamic one) ignore the solar year altogether and just declare twelve lunar months to be a year.

When an Alternative Calendar is used to measure the progress of "days", it's common for characters to use Microts as smaller, more manageable units of time. In Sci-fi settings, these calendars are frequently used across multiple worlds, becoming Standard Time Units.

If Alternative Calendar is used in Science Fiction with Earthian years, it may mean that the work takes place After the End or something else that Hit So Hard The Calendar Felt It or that everybody have Outgrown Such Silly Superstitions and chose something significant for their reference point.

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Examples:

Anime and Manga

Code Geass takes place in the year 2017 a.t.b ("Ascension Throne Britannia"), counting from the establishment of the first Celtic King in the Britannian royal line, rather than the birth of Christ.
- If converted into AD*
  a.t.b is 54 years before AD
  , much of the show's events occur in 1963. Technology developed more rapidly in this universe due to the discovery of sakuradite.
Every Gundam series except Gundam 00 uses this; indeed, the various calendars are a common way of differentiating between Alternate Universes.
- As a direct result of Gundam 0079's success with the Universal Century calendar, this trope can also be found in the majority of Real Robot series produced since.
- Apparently, somebody once did the math based on the few instances where days of the week are given for certain dates and found out that UC 001 corresponds to 2047 AD.
- The Gundam Wing sequel novel Frozen Teardrop features the first Alternate Calendar within a series: while the Earth Sphere uses the AC (After Colony) calendar, Mars uses the MC (Mars Century) calendar, with years roughly double the length of Earth's. This makes it more than a little confusing as to when exactly Frozen Teardrop takes place in relation to the main series.
Battle Angel Alita (Gunnm in Japan), particularly the space colonies in Last Order uses a calendar dating from the launch of Sputnik in 1957 AD.
Kiddy Grade takes place in "Star Century 0165".
Magical Girl Lyrical Nanoha uses the old and new Mid-Childan calendar, with the new one starting sometime after the end of the Ancient Belkan War, when Magitek replaced physical-based weapons as the standard due to the devastation the latter caused. Interesting in that they also use the regular Earth calendar, thanks to worldhopping. The series began in year 65 of the new Mid-Childan calendar.*
A common fan theory estimates that the New Calendar began around 1942 by Terran reckoning, so 0065 NC corresponds to 2006 CE.
Sora Wo Kakeru Shoujo is set in the year 311 O.E., or the Orbital Era, counting from when humanity began living in a space colony orbiting Earth as well as on the planet itself.
Library War takes place in year 31 of the fictional Seika era (see below on the Japanese era system), thus providing a date that fits into a modern calendar system while leaving no way to connect it to a real date.
The Magic World of Mahou Sensei Negima! has its own Imperial Calendar to differentiate it from the real world. The series takes place at around the year 1018 of that calendar, which is around 2003 of ours.
Hyper Police takes place in the "22nd Year of the Holy Century".
Soukou No Strain has S.E., although we aren't really sure what it stands for. (According to the dub, it's Standard Era, but how canonical that is is debatable.)
In Legend of Galactic Heroes, both the Galactic Empire and the Free Planets Alliance use different calendars. The story takes place in about the 35th century AD. The Empire had previously established their own timescale, and when the FPA was formed, they in turn split off from the Empire's calendar.
- Actually, the calendar used by the FPA was established when humanity first left Earth. When the Empire was founded, they created a third calendar which began on the date of the first Emperor's coronation. The FPA did not create their own calendar but instead adopted the one that was abandoned by the Empire.
So Ra No Wo To starts in the spring of 271 A.P. and ends in February the following year. While not explicitly stated, it is presumably 271 year after the huge world war ended.
Dragon Ball takes place on a planet called Earth, but they follow an alternative calendar (the year Goku meets Bulma is given as 749), the life-forms are full of Talking Animals, and it's home to the titular wish-granting Dragon Balls.
Tiger & Bunny begins in 1977 N.C. (New Century). The only hint as to the real-world date would be the show's pilot, which makes mention of the 22nd century.

Comic Books

In a Calvin And Hobbes strip Calvin says that the U.S. was founded roughly around 200 B.C. - "Before Calvin".
The world of Nikolai Dante generally uses the Gregorian Calendar, but refers to each year as 'Year of the Tsar'.

Fan Works

The Central Shadow Realm fanfiction series counts from a randomly-chosen point during the construction of the titular city; those living in the city admit it's arbitrary. The months and days don't have names; dates are given as "The (x) day of the (x) month". The first story, Shadow Realm: Fifteen, took place in 5178.
The Magick Knight: Guardian Seed fanfic uses the "Era of Magick" calendar, originally adopted by the "League of Nations" after the discovery of the Plants to the west and the proliferation of Adepts. Progress through the year is measured in the seasons, Winter (December-January-February), Spring (March-April-May), Summer (June-July-August) and Autumn (September-October-November). The story takes place in 70th Year of the Era of Magick.

Film

Played for laughs in Santa Claus Conquers The Martians:
Chochem: What time of year is it now?
Kimar: It is the middle of Septober.
Downright bizarre in Waterworld, where the Smokers speak of "lunars" of time.

Literature

Chung Kuo by David Wingrove has erased all traces of the world's real past, including the birth of Christ
The Draka: The Draka introduce a new calender after winning the Final War.
The Foundation series by Isaac Asimov. The calendar begins thousands of years before the stories, with the founding date of the Galactic Empire, and later there's a new calendar that begins with the first year of the titular Foundation. However, it's averted in Asimov's book The End of Eternity, even though an Alternative Calendar would make just as much sense as it does in the Foundation series. In that book, characters will refer to "the 78th Century", "the 482nd Century", or even "a Century in the 30,000s" (i.e. 3 million years after the present), and these are dates expressed in terms of our very own Gregorian calendar. The character Cooper is from the 78th century, and although he's familiar with the idea that the year he comes from is some number in the 7700s, he apparently needed an explanation to understand why there were centuries before the 1st Century. With situations like this, you can see why this trope makes sense in fiction: because if you take a long enough view, it seems very probable that it will become Truth in Television.
- A couple of Asimov's stories referred to a calendar starting with 1945 - Nuclear Age, that is. In one of them it is a plot point, because an angel manages to delay the end of the world by pointing out that there it must come on a certain Earth day, and without a single calendar for all, there is no such thing. So, Satan decides to strike back.
- Another Asimov story involves people from "our world" encountering people from a German-speaking parallel Earth which counts their calendar nach Hitler - "after Hitler". Presumably their calendar begins in 1889 (Hitler's birth year), 1933 (when Hitler came to power in Germany) or the year Hitler died in that reality. The story takes place far enough in the future that the protagonists don't evince the horror that we might at the prospect of a world where the Nazis won.
  - The year turns out to be "Zwei tausend drei hundert vier-und-sechzig nach Hitler" (i.e. 2364 NH). Apparently that's long enough that their culture is no longer Nazi-like; they readily abandon their claim to an uninhabited third Earth simply because the protagonists' world got there first. The introduction of the Nazi-descended world may be simply a pun of sorts on the story title: "Living Space" ("Lebensraum" in German).
- One of the later Foundation novels is notable for having a historian discuss calendars with a companion who's looking for Earth-that-Was. The historian has determined that, based on his studies, the planet must have a yearly cycle of 365 days with each day being 24 hours, based on the system that been in use by humans since time immemorial. None of the planets in the catalog match those criteria.
Steven Brust's Dragaera novels use "in the Xth year of the reign of Empress Zerika" (or whoever's on the throne). For more formal dating, they also have a more complicated system of Great Cycles, Cycles, Reigns, Phases, Turns, and Years; each (save "reign") is exactly 17 of the next stage down, and the entire calendar starts with the formation of the empire. Weeks are five days.
David Eddings' The Belgariad uses the Alorn Calendar, which measures time in years since the cracking of the world. Additional calendars used by other nations are occasionally mentioned throughout the series.
Frank Herbert's Dune dates events from the establishment of the Spacing Guild's monopoly on space travel; the original novel Dune begins in the year 10,191 A.G. In addition, in God Emperor of Dune, it's implied that the (3000-year) reign of the titular God Emperor Leto II has in effect become a calendar. The prequels written by his son and Kevin J. Anderson avoid going into the pre-Guild calendar by having the events simply be compiled and retold by Irulan, so naturally she uses the same calendar, with the dates being B.G. (Before Guild).
Brave New World reckons its time from the completion of Henry Ford's first Model T*
AD 1908
("in the year of our Ford"), as part of its satire of the post-Industrial Revolution world. Ironic, considering that Ford himself said "History is bunk," and is quoted as such in the book.
The glossary of Robert Jordan's The Wheel of Time books states that the world is on the third or fourth calendar since Rand, in a previous life, broke the world. Ten-day weeks also appear at some point, though the first few books used seven-day weeks. There are also months, but most people ignore these and just go by seasons.
Ursula K. Le Guin has scores of different calendars:
- Gethenians count years backward and forward, with the current year always being year one. The more exact dates are determined by their relation to some historical events.
  - But they are kinda strange anyway.
- Werel has a 400 days' lunar cycle and a solar cycle of 60 moonphases. Year, lifetime, what difference?
- Curiously, the fantasy book Gifts uses the Gregorian month names, but this might be a Translation Convention, and it's not clear if it's meant to take place on an alternate Earth or another planet.
- Rokanan has years twice the length of an earth year (or, rather, the standard year used by the League of Worlds). Some regions use two twelve-month (or however many they have) cycles to make up a full year.
Perdido Street Station and other works in the Bas-Lag Cycle use rather bold-faced names for the days, such as Skullday, Chainday, and Fishday.
- And this is only for the city of New Crobuzon, alone. In the floating city Armada, they have completely different names for months and days of the week .
H. Beam Piper's Terro-Human future history (e.g. Little Fuzzy, Uller Uprising) uses Atomic Era dating, starting the year zero A.E. at 2 December 1942 by the C.E. calendar (the date of the first self-sustaining nuclear chain reaction). Characters occasionally speak of Nth Century Pre-Atomic dates to refer to dates before that time.
Austin Tappan Wright devised a fictional calendar for Islandia, which is explained in more detail in Basil Davenport's treatise on Islandian history. When Islandians found out about the Gregorian calendar, they liked the idea, but adapted the month names so they would have the right seasonal connotations for the SOUTHERN hemisphere. Thus, "Octen" and "Noven" are roughly the northern April and May.
Terry Pratchett: Most places on the Discworld use a 800-day calendar with eight-day weeks (the usual seven used in our world, plus Octeday). The year is divided into two half-years of thirteen months each (one month two weeks long, the other twelve four weeks long): Ick, Offle, February, March, April, May, June, Grune, August, Spune, Sektober, Ember, and December. The idea's also played with in Wyrd Sisters, which mentions several different calendars used by different countries and kingdoms, including one used by the Theocracy of Muntab, which counts down instead of up. No one's sure why, but most agree it's not a good idea to be around when it hits zero.
- It's usually only wizards and astronomers who think of a year as 800 days long. To the average Discworlder, concerned with practicalities like harvests and weather, a Disc "year" lasts for one turn of the seasons, which takes only 400 days.
- Ankh-Morpork itself has restarted its calendar several times in its history, though dates are usually given in Unseen University's Ankh-Morpork Years. At least one of these restarts is a Retcon to make up for inconsistent dating.
- Years and centuries are often named rather than numbered on Discworld. Sometimes there's a reference to a year like "1784," but more often, years are referred to as "the Year of the Diffident Squid." And the whole continuity has just recently completed the Century of the Fruitbat.
  - Careful reading of the books seems to imply that the numbered years are closely aligned with our own, and it is currently 2010 on the disc as well.
- The reason for the hodgepodge of real and fictional month names appears to be that, as with so much on the Disc, Sir Terry gave months names based on what worked in that scene, and then Stephen Briggs came along later and worked out how it all fitted together.
J. R. R. Tolkien's Legendarium (The Lord of the Rings etc) has several calendar systems for the different peoples. Used most commonly in LotR, the Hobbits' Shire Reckoning uses a reformed calendar in which the week and the year are synchronized, and hides it behind a Translation Convention. The Dwarves appear to use a different calendar, leading to such things as Gandalf in The Hobbit saying to Thorin "on the twenty-first of April, a hundred years ago last Thursday" when, in the Shire Reckoning, the twenty-first of April always falls on a Friday.
In Jack Vance's The Demon Princes, the year zero corresponds to the year 2000 A.D. Therefore the first book taking place in 1524 means that it is really 3524 A.D.
- The fact that the new calendar began in 2000 A.D. is only given in a footnote.
- A foot note in the 4th book, written after Apollo XI, retcons the epoch from 2000 to 1969.
The "Common Era" in Alexei Panshin's Anthony Villiers novels apparently is reckoned from the launching of Sputnik I in 1957.
In A Deepness in the Sky by Vernor Vinge, interstellar traders have done away with not just years and months, but also with days, hours and minutes. All timespans are measured in (appropriate powers-of-thousands of) seconds, with dates simply being the number of seconds since the UNIX epoch (with some unspecified but explicitly mentioned relativistic frame corrections) — though in-universe it is most commonly thought that the zero-second was at the first moon landing. Settled planets all have their own calendars. One way the traders know they have stayed too long is when they start using the locals' calendar.
The Honor Harrington books count from 2103 AD (Gregorian) as 1 PD (post-Diaspora), the year that the first generation ship left Earth. The events of the books take place in the early 20th century PD (41st century AD). Planets set their years as "Year X AL (After Landing)", with days and months set as appropriate to planetary conditions. This means that a given planet will retain at least two calendars: The "Standard" (PD) year, & the "Local" (AL) year. The ones that are part of a multi-planet polity will track the capitol planet's year AL. Yay for computers.
- The religiously-founded planet Grayson averts this, using the Gregorian calendar even though it doesn't even remotely track with local conditions.
- The Kingdom of Manticore itself starts off with 5: Terran, Manticoran (the official one for the kingdom), Gryphon, Sphinxian (the latter two for their own seasons and days), and Medusan. Each additional planet that's added to the Kingdom adds another one.
- Nearly every mention of any amount of local years will be accompanied by "XX in T Years" to make sure we Earthlings have some frame of reference. Justified in this case: because of the kingdom's multiplicity of calendars, you'd expect to see them using a common frame of reference. Those people who have to deal with multiple planets and other nations (with their own calendar systems) are probably routinely doing conversions in their head automatically. You can see the same thing with people who use both metric and imperial measurements.
  - Since Manticore is a merchant empire, it's extremely common for Manticorans to move to and from different planets, so local calendars are usually of little use to those new nomads. The series being Military Science Fiction, it concentrates mainly on exactly this type of character, so the series generally dropped references to the local calendars and is now written mostly around the P.D. calendar.
In The General series, dates on the planet Bellevue are calculated from "the Fall" of the interstellar Federation to which the planet once belonged.
David Foster Wallace's Infinite Jest is set in a not-so-distant version of America after the adoption of a calendar system that names each year after a corporate sponsor. Most of the book's plot takes place during the Year of the Depends Adult Undergarment. It's that kind of book.
In Edgar Rice Burroughs's Pellucidar, the immobility of the central sun prevents any sort of celestial timekeeping, leading the protagonists to proclaim (unconvincingly) that there is "no such thing as time" at the Earth's core.
- Evidently, the possibility of estimating time's passage by means of tides, heartbeats, sleeping habits, when the hero needs a shave, or any other ground-based phenomena never occurred to Burroughs.
  - Burroughs claimed that time in Pellucidar passes more swiftly for you when you're exerting yourself more. At one point, the hero and his friend were separated and the hero had several weeks worth of adventures: fighting, running, lots of hard work. His friend merely strolled back to their lodgings, so only about an hour passed for him. It's a pity the "I'm not making this up" category has been disabled, because this nonsense deserves it.
- In John Carter of Mars, it gets rather more rational treatment: he believes Dejah Thoris has died in "The Gods of Mars" because it has been a year — but he has forgotten he's on Mars.
Brian Caswell's Deucalion uses a metric time system of years AS (After Settlement), dating from first landfall on planet Deucalion (Earth still uses conventional time).
In Melanie Rawn's Exile series, the year is divided into 36 10-day weeks, each with a name and a patron saint, plus a handful of feast days for a total of 366 days. This replaced a system where every day had it's own patron saint. Needless to say, this particular setting has a lot of saints.
The Dragonlance series. There are seven days each week and twelve months each year. But each day and month have eight different names, each from a different culture. The titles of the Chronicles trilogy, in the Gregorian calendar instead of elven are: Dragons of October, Dragons of January, and Dragons of March. The War of the Lance lasts less than a full year. And now you know.
In Stephen King's The Dark Tower, Roland's world uses different names for the months.
On Gor each city-state has its own calendar, counting Year X of City Administrator Y's reign or some other locally-important event. To standardize, most city-states also count in years C.A. - Contasta Ar, "from the founding of Ar," the largest city-state. Actual book quote:
Chronology is the despair of scholars on Gor.
In Star Trek: Department of Temporal Investigations, alongside multiple human calendars including Christian, Islamic, Hindu and Mayan examples, the chapter headings include dating systems from many Star Trek cultures, including Vulcan, Andorian, Cardassian, Klingon, Deltan, Tandaran and Risian. Most of these alien calendars have been plotted out in full by the author in his annotations. Other Star Trek Novel Verse books have given dates in mostly consistant Klingon, Vulcan, Romulan and Andorian calendars, but this is probably the first time the entire calendar has been plotted for so many races.
The Witcherworld knows at least two calendars, human and elven. The world's actual year length appears to be used by the human calendar, while elven year's length appears to be some two thirds of it. Both share equinoxes and solstices as points of reference.
A Song of Ice and Fire is set in the second to third century after Aegon's Landing. The seasons are random in length, but are much longer than the years, making it uncertain what a "year" actually means.
- Probably related to a complete rotation of the sun, our own calendar is measured as such, rather than the passing of seasons.
The various chapters of Out Of The Dark provide the date in both the Gregorian and the galactic standard calendar. The epilogue introduces a third calendar, replacing the Hegemony's calendar with the notation "Year 1 of the Terran Empire.
The Animorphs prequel ''The Hork-Bajir Chronicles" gives Earth dates (the late 60s), Andalite dates (a year in the 8000s with decimals attached), Hork-Bajir dates (simply a reference to the current season) and Yeerk dates (a reference to generational cycles).
In The Kingkiller Chronicle, a week is called a "span" and has eleven days. It's unknown how many spans there are to a month, nor months to a year.

Live Action TV

Though the original calendar system is not explained, the 2000s Battlestar Galactica makes reference to a (new) calendar system after the exodus from New Caprica. After passing a guilty sentence on a Cylon collaborator, the date is stated as the "third day of the Second Exodus."
Caprica introduces a twelve-month calendar using the same names as the Roman calendar: Ianuarius, Februarius, Martius, Aprilis, Maius, Iunius, Quintilis, Sextilis, September, October, November and December.
- The Roman months Quintilis and Sextilis were renamed after Julius Caesar and Augustus Caesar respectively.
In the Doctor Who episode "The End Of The World", the Doctor claims that the year to which they've traveled is "5.5/apple/26, five billion years in your future".
The Red Dwarf episode Better Than Life had a newsreader announce the news for Friday the 27th of Geldof.
- Channel 27 might not be based on Earth.
Star Trek's famous stardates, though these are much more complex than most. They also use straight-up AD/BC.
- The writers of Star Trek: The Original Series made up stardates as they went along without any sort of linear progression or continuity. Early fans who tried to use them to reconcile episodes' production order and air date order tended to end up as inmates at Tantalus Colony. When an interviewer asked James Doohan how stardates worked, Doohan replied (paraphrasing), "Sorry, even Scotty couldn't figure that out."
- Parodied in Futurama:
  Zapp: Captain's journal. Stardate: uhhh...
  Kif: (sighs) April 13.
  Zapp: April 13... point two.
- Averted in Enterprise, which still used the Gregorian calendar.
- Also averted in the new Star Trek movie which uses stardates that are just different ways of saying the date than we use now. Stardate 2248.42, for example, would be February 11th (the 42nd day of the year), 2248 AD.
- For the three "middle children" of the series (Star Trek: The Next Generation, Deep Space Nine, and Voyager), the stardates have some coherency. They range from the 41000s for TNG's first season to the 54000s for the last season of Voyager. Originally, the 4 stood for the 24th century, the 1 stood for the first season, and the last three numbers were given out linearly to the episodes of a season, with the digit after the period representing a fraction of days. Deep Space Nine started during TNG's run and used similar stardates, so the 4 had to lose its "24th century" meaning and the second number lost its season meaning, leaving a seemingly coherent in-universe system: a thousand stardate units equal a year. Voyager continued this, and so did the TNG movies. While this may seem like it makes sense, there are countless contradictions, errors, and inconsistencies.
  - Notably, this system would place Stardate 0.0 at 1st January 2323 AD... some thirty years after the events of Star Trek VI. Which isn't necessarily a problem, if the Federation changed the Stardate system at some point along the way, like what we know they did with the warp speed scale.
- The Deep Space Nine episode "Soldiers of the Empire" reveals that Klingons use a stardate system of their own (day in the year of Kahless).

Play-by-Post RP Gs

In Shadowside, the world switched over from 2007 AD to 00 AED, "Years After Evos Dawn". It is currently 03 AED.

Tabletop Games

The world of Dominaria in Magic: The Gathering has an undefined calendar within the year, though years are given in several different scales. A nation famed for its education and archaeologists is even referenced in the name of the most well-known calendar — Argivian Reckoning (AR), which places Year 1 on the year Urza and Mishra, leaders of the two sides of the Brothers' War, were born.
The Forgotten Realms setting was hit with it rather hard. The main calendar on Faerun is Dale Reckoning (DR), aka Freeman's Reckoning (FR). Present Reckoning (PR) is zeroed at the Time of Troubles. The Roll of Years names years (i.e. "Year of Rogue Dragons") according to the prophecies of the seers Augathra and later Alaundo covering -422 DR to 2163 DR — which at least helps to stitch local numerical calendars together. There are even different versions of this, where the orcs started out the year in a blaze of conquest and by the end of the year had been kicked back into their holes once they'd pissed enough people off. They call that "The Year We Went Too Far". The Forgotten Realms also uses a ten day week (called, appropriately enough, a tenday) and several feast days, holidays, and other events that vary from nation to nation throughout the year.
On Eberron a year consists of twelve months (each named after one of the twelve moons) and a week of seven days, seeming like they just came up with with fancy new names for the months and and days to seem exotic, however the months are only 28 days each, making all characters a couple of years younger in real time than they seem. Interestingly (or not, depending) there was originally one more moon, implying before the appropriate cataclysm the year and lunar cycle used to be much closer to earth's.
A year on the planet Mystara is 336 days long, allowing for a handy calendar of twelve 28-day months. There is no distinction between the lunar and solar calendars, as new or full moons always fall on the same dates in each year. Each country has its own names for the 12 months; the chief exception is that of the shadow elves, whose year consists of 24-day months, each named for one of their religion's 14 Verses. (Being subterranean, these elves don't need to synch their calendar with celestial events, and keep track of dates only to schedule their religious ceremonies.)
- Inside Mystara, the Hollow World's stationary inner sun doesn't provide for conventional day/night or seasonal cycles. Rather, dates are tracked by the orbits of the major flying continents.
In Warhammer 40000, the Imperium still uses the Gregorian calendar, but the presentation of years is different: the current year as of this entry in the Gregorian calendar, 2008, would be written as "008.M3". However, the internal workings of the calendar is a bit different than the modern Gregorian Calendar. Take for example December 24th 2008: the using the Imperial dating system, this would be 0979008.M3 . The first number (0) means that the dated event took place in the Sol system, allowing perfectly accurate dating. The more inaccurate that dating becomes, due to distance from Earth and lack of information, the higher the first number will be. The next three ciphers (979) the fraction of the year that has passed*
though this is only really used in bureaucratic records
. The next three ciphers (008) is the year. And the ending (.M3) is the millennium.
Exalted uses a dating system starting with the founding of the (second) Realm, and has a rather different calendar inside each year also. The year is divided into 15 months (ascending, resplendent, and descending Element, where Element is one of Air, Earth, Fire, Water, Wood (not in that order)), plus five days of Calibration, when spirits are at their strongest and the gods are all partying.
- Well if you are going to pedantic about it (and I am) Calibration is not part of the year, the five days of calibration do not belong to either the year before or the year after.
- The various different regimes use different calendars, many of which will be in use at the same time in different places. The nominal "start of game" year would therefore be Realm Year 768; 4878 Dawn of Autochthonia; Year 4999 of Our Everlasting Glory and the Year of the Mouse in the Bronze Era of the 11th Epoch the Dragon-Blooded Shogunate.
- Being Exalted, it also has an extremely alternative calendar — the Calendar of Setesh, which not only tracks time in the Underworld, it actually cements the concept of time passing in a linear fashion down there.
- An interesting detail is that despite having 425 days, each of 25 hours, in the year, a sixteen-year-old in Creation is functionally equivalent to a sixteen-year-old on Earth.
The German P&P System Das schwarze Auge(The Dark Eye in the US) has several calendar systems. The most common (at least in the Middlerealm) is one that uses a year of 365 days with 12 months of 30 days each, named after the Twelvegods (Praios, Rondra, Efferd, Travia, Boron, Hesinde, Firun, Tsa, Phex, Peraine, Ingerimm and Rahja). The five days after Rahja are called "Nameless Days" and are dedicated to the Nameless God (you better stay at home during those!). The year 0 can be either the fall of the city Bosparan, the year Emperor Hal (or Reto) ascended to the throne, etc. And that's only for those believing in the Twelvegods...
- There is only one god and His name is Rastullah.
- everybody besides the deciples of Rastullah above uses the calender of the Middlerealm, just with other names for the months. And even the deciples of the Onegod observe the Nameless Days.
Averted in BattleTech where thousands of planets all still use the current AD system. The main universe storyline runs from from 3020 to 3135. Individual planets have 24 'hour' days (where an hour can be as little as 30 minutes on some fast-spinning planets) based on their own rotation, and years based on their own orbital path. But Human civilization as a whole maintains the earth-based AD Calendar, with 0 hour based on Greenwich Mean Time, including leap years!
In Talislanta, a calendar year consists of seven months, each month lasting for seven weeks of seven days. Talislanta has seven moons, so it makes a certain degree of sense.
In Paranoia, the days of the week are as follows: Oneday, Twosday, Threeday, Fourday, Fiveday, Sixday, and Mandatory Inspection Day.
- And the year is 214. See, The Computer decided that 214 makes things sound nicely established but not too old, so the year is always 214.
On Athas, the world of Dark Sun, the calendar system is a merchant's calendar that is used by all of the known Merchant Houses to help with trade, which is based primarily on the orbits of the two moons, Ral (11-years to return to the same orbit) and Guthay (7-years to return to the same orbit) which each year for each moon designates a certain word used to define the year (Tyr's Free Year 1 is the 4th year for Ral, designated "Priest", while it is the 5th year for Guthay, designated "Defiance", so it is the Year of the Priest's Defiance). The calendar year is typically broken into 15 months comprised of 5 weeks each 6 days long (30-day months). A rather complex calendar system overall. The actual year numbering is either based on the King's Ages (older system), or since Tyr's revolt (Free Years).
The Iron Kingdoms use a calendar where 7 days make a week, 4 weeks make a month, and there are 13 months per year. Furthermore, the days are not named, but the weeks are, and the whole thing is based around the cycles of the planet's 3 moons. Every 3 years, all 3 moons are new on the same night, which makes an extra day on the calendar. This 'Leap Day' is celebrated in a festival called The Longest Night.
In Warhammer Fantasy Role Play, Imperial calendar counts the years from the coronation of Sigmar, an event marking the birth of the Empire (canonically the game is set in 26th century). The year is 400 days long and divided into 12 months with six with intercalary holidays, while weeks are eight days long. Other nations use similar calendars, usually numbering the years from the founding of respective state (e.g. 977 IC for Bretonnia, 1524 IC for Kislev). And the calendar of High Elves is divided into epochs of variable length, each one referring to the reign of the particular Phoenix King (very similar to the Japanese eras). Wood Elves, Ungols and other nomadic tribes use various lunisolar calendars.
In Dragonlance people started numbering up from zero after the Cataclysm, so the abbreviation was A.C., anything before the Cataclysm was labeled P.C., and after what is sometimes called the Second Cataclysm in Dragons of Summer Flame, S.C. was used for a while, but was later dropped in official supplements and novels, going back to using A.C.
Subverted in Ravenloft, where the calendar works like Earth's, despite the setting's having been cobbled together from bits and pieces stolen/copied from other worlds. The Barovian calendar predominates for the numbering of years, while the names of months would seem to have been imported from Mordent.
7th Sea has a calendar that, like everything in 7th Sea, is strongly reminiscent of the one used in 18th century Europe with just enough cosmetic changes to make it more of an Expy that a direct reproduction of the real world.
In Traveller the calender of the Third Imperium starts at the beginning of the reign of Cleon the Great. There is an official Imperial Office of Calendar Complience to insure that this calendar is kept up between worlds. Some have said that this doubles as a Secret Police.

Video Games

The Elder Scrolls games have a system which is actually our own calendar system, but with different names for the months and days of the week. October becomes "Frostfall", Saturday becomes "Loredas", and so forth. The number of the year is determined by the amount of time since the beginning of that particular Era. Oblivion begins on the 27th of Last Seed (August) in the 433rd year of the Third Era, which began with the unification of Tamriel and founding of the Septim Dynasty and ended with the events of the game. Skyrim begins on the 17th of Last Seed in the 201st year of the Fourth Era, which began following the conclusion of the Oblivion crisis.
Final Fantasy Tactics uses a calendar based on the Western Zodiac. This was changed in the English PSX version to the Gregorian calendar, but kept in the PSP version.
- Final Fantasy Tactics A 2 has its own calendar, which is the regular one but with different names and twenty days for every month.
Final Fantasy VII uses Gregorian names for months, but February has at least 30 days. And the number of the year gets reset after each 2000-year Greek-letter "era", so 1999 of the Mu era was followed by 0000 of the Nu era.
Freelancer measures the years as "B.S." ("before settlement") and "A.S." ("after settlement"), with the arrival of the Liberty to Planet Manhattan as its zero point; days and months, meanwhile, are measured with our Gregorian calendar and our 24 hour days. In the popular Discovery modification, the calendar in House Gallia uses AGS, or After Gallic Settlement.
The Ogre series has the Zeteginean calendar, which corresponds to the Gregorian one for the purposes of allowing the player to choose a birthday, but is otherwise different.
Guild Wars has its own set of calendars, one for the Canthan continent, and one that is shared by both the Tyrians and Elonans. The Elonan calendar also has a zero year some two centuries before the Tyrian calendar.
The freeware space exploration game Noctis puts the player in control of a cat-like alien creature called a Felysian, and employs the calendar of his civilization. For example the base unit, called epoc, corresponds to about 32 human years, but the smallest unit, the triad dexter (a billionth of an epoc) is "incidentally" equal to a second.
Kingdom of Loathing uses a 12-month calendar in which each month has 8 days. The first day of each month is when Ronald is full. The holidays that affect the goings-on of KoL are: KoL-specific holidays, which pop up only on their day of the KoL year; our holidays and other observances, which occur when they occur in our year (for example, our Friday the 13th causes things to happen there, even though KoL months don't have that many days to begin with); and parody holidays, which tend to happen both on a specific date on the KoL calendar and on the date of the parodied observance.
- This occasionally creates interesting scenarios where real holidays and their ingame versions intersect, including one memorable instance where Christmas (or Crimbo, as it's known in game) and the in-game Halloween coincided, or Drunksgiving, where the in-game holiday 'Feast of Boris' occured on the real-life St. Patrick's Day.
Played in the MMORPG EVE Online. One of the EVE chronicles discusses how the four empires decided on a common calendar system. They ended up using the Gregorian calendar, and they date of the decision became January 1, year 0. The months of the EVE calendar match up with those in real life. Years are done a little differently; the year of the game's launch, 2003, corresponds with the in-game year 105. 2008 is referred to in-game as year 110.
- Averted in the Amarr Empire, where they still count from Anno Domini (As of 2009, the current in-game year is 23347 AD and 111 YC). Also, it seems reasonable to assume that the other empires have their own calendars. One has been mentioned for the Gallente Federation - The Age of Rouvenor (AR), with Year 0 established when king Doule Dos Rouvenor III rose to power in 21714 AD.
In the StarCraft expansion, the United Earth Directorate uses a different calendar, and the present year, 2501 AD (The original game used the Gregorian calendar), is marked as 872 GD. It's not explained what significance the year 1629 AD has to be year 0 on the UED calendar.
- It was the year the dutch astronomer Christiaan Huygens was born: given his contributions to the fields of mathematics and astronomy, and the fact that he was one of the first to speculate about the existence of extraterrestrials, it may be viewed as a starting date as good as another, given the kind of universe that is Starcraft.
  - Seeing as how the UED was formed out of the UPL due to the discovery of aliens in the koprulu sector, it would seem that they chose the date because of Christiaan Huygens speculations on extraterrestrials. Which would make for the fun realization that this calendar is fairly new, as aliens had only been discovered several months before the UED invasion.
Syndicate gave it's dates in N.C., simply standing for New Calendar. This added somewhat to the corporate dystopian feel of the game.
While the structure is unknown, dates in the two pre-Wii Paper Mario games are written with triangles and circles and stuff instead of names or numbers.
The Harvest Moon games use years made up of four thirty-day seasons. The years themselves count up from the start of the game, which is usually when the player character arrives in town. The Rune Factory spinoff series takes this one step further by combining Saturday and Sunday into "Holiday", meaning that every day of the year will always be on the same day of the week.
- In the Wonderful Life games, there are still four seasons, but only ten days per season (with no designation for days of the week).
- Averted in My Little Shop, which, like Animal Crossing, uses your Wii's system calendar.
Homeworld uses four calendars: Kharakian Dating System (KDS), Before Hiigaran Landfall (BHL), After Hiigaran Landfall (AHL), and Galactic Standard Year (GSY). The first three were used exclusively by the Hiigarans, while GSY, as implied by its name, is used by all other races. BHL and AHL are used in the same manner as our BC and AD, except for the use of year 0. KDS started in 1216 BHL, while GSY began in 9510 BHL. Confused yet? If so, then I would not even attempt to read this timeline.
- Strangely enough (or, more likely, so as not to confuse the players even more), all four calendars have the same length of the year.
Super Robot Wars counts, since it has Gundam. Here are a few examples
- Super Robot Wars Alpha series takes place in the New Western Calendar
- Super Robot Wars Compact, Super Robot Wars Compact 2 and Super Robot Wars Compact 3 takes place in 00XX
- Super Robot Wars Destiny takes place on Space Year 145
- Super Robot Wars Judgement takes place on Cosmic Era year 70
The Dragon Age universe actually has multiple calendars. The most widely-used one is the Chantry calendar, which groups time into 100-years Ages, and which starts with the founding of the Chantry. In the year before a new age is set to begin, the Chantry look for portents to determine the name of the upcoming age, with the name reflecting major events that will happen during those hundred years. The Tevinter Imperium has its own calendar that starts with its founding, and the elves have their own calendar that begins with the founding of Arlathan. See here for more info.
Soul Nomad uses the Tamaito calendar, with the game taking place in the year 800.
In both Xenogears and Xenosaga, humanity switches to the "Transcend Christ" calendar, with AD 2510 as TC 1.
EV Nova years use the suffix "NC", for "New Calendar". The game starts in 1177 NC; 0 NC was 2780 AD, the year FTL inventor Omata Kane died.
In the X-Universe, year 0 Argon Stardate is 2170 AD, twenty-four years after a Terran warfleet lured insane terraforming drones away from Earth, trapping themselves and the terraformers in the X-Universe. The fleet established their own society (called the Argon, after their leader Nathan R. Gunne), and erased all mention of Earth from their histories to prevent people from inadvertently leading the terraformers back to Earth. The games take place over 700 years later.
- The Terrans still use Earth's calendar, understandably.
Dwarf Fortress has a variation on the Gregorian calendar, although with the names of minerals for the months.
In Sid Meier's Alpha Centauri, a subversion: The game begins in AD 2101 and you presumably continue to follow the Gregorian calendar, but for whatever reason you now them "Mission Years" (MY). One imagines that this is rather confusing to anyone on Planet who cares to think about it.
Ever Quest II takes place in the year 500 A.K. (After Kerafyrm,) starting the new calendar after the famous day when the powerfully insane dragon was woken up by mortals and went on a rampage against his own dragon-kind in the original EverQuest. Things get complicated when you try to compare it to the calender that the first game used: There was none. Each server had it's own in-game date, and always fluctuated during downtimes like patches and such. However, it doesn't stop there. During those 500 years between the two games, the Ogres rose up and built a new empire that tried to take over the world. Their own records of the 2nd Rallosian War are recorded in their own established calendar system. Over on the continent of Kunark, the evil Lich Venril Sathir built a new Sathirian Empire, using the ancient Sathirian Calendar themselves. None of these established calendars have any overlapping date of reference to pinpoint exactly when they take place on a universal calendar that the player can reference. This was done intentionally by the developers so they would have some leeway to fit in new events that happened during the 500 year period without conflicting with anything.
The Metroid series uses locally-defined "cycles" for its calendar system. The only point of reference given is that the Galactic Federation officially formed in 2003 AD.
According to Curiosities of Lotus Asia, Youkai from Touhou have their own calendar, which is adjusted to natural phenomena (such as earthquakes or the blooming of bamboo flowers) and extended life spans. Few of the creatures actually use it, though.
In Half Minute Hero, the 1st year of the Goddess Era marked the first time the Time Goddess descended upon humans. Every game mode takes place at a different point in the Goddess Era.

Web Original

In Pokegirls, the world mostly uses the Gregorian system of days, months and years, although the zero-year is centered on the year of Sukebe's death, 2002. Thus, the canon 'modern era' of Pokegirls fiction, 300 AS, would be 2302 AD.
In the League of Intergalactic Cosmic Champions, the year 3000 is the Year 0 of the New Calendar.
Orion's Arm uses the Tranquility Calendar given below. The setting extends to 10601 AT, After Tranquilitate).
Tasakeru: The Sankami calendar was intentionally designed to be convertible to Earth-calendar format.
The Chaos Timeline has the Sixtine Calendar, and ends up using 100 seconds per minute.
Imperium Nova uses a 400-day calendar with ten months to keep things simple. Each galaxy numbers years based on the ascendance of the current Imperial house, which like the Japanese system can make long term dating difficult in certain galaxies.

Western Animation

In an episode of Doug, an extended fantasy sequence involving Quailman has a villain based on Mr. Bone, known as the Rulemeister, wipe out weekends from the timeline, forcing school students to attend school every single day (with no weekends on which to relax on). After Quailman defeats the Rulemeister by dropping a Logic Bomb of sorts on the Rulemeister, Quailman brings back weekends and adds an eighth day, "Funday," to the week, something that the narrator erroneously describes as "slowing down the Earth's rotation."
On Jimmy Two-Shoes, all the months are named after Lucius. (Lucember, Lucapril, etc.)
One Simpsons Treehouse of Horror episode had a flashback to the thirteenth hour of the thirteenth day of the thirteenth month. Lampshaded by the fact that misprinted calendars were mentioned.
Homer: Lousy Smarch weather,
On Family Guy, NBC invented Catillsday so they could have another Dateline.

Real Life

Academic circles are prone to replacing BC/AD with BCE/CE ("Before Common Era"/"Common Era") to be more secular. Generally speaking a cultural non-starter with the general public, also liable to get certain Christian groups angry. The actual purpose behind the idea isn't really to avoid dating by Jesus*
no academic is dumb enough to pretend that's actually changing, what with the numbers being exactly the same
, but to not have to imply acceptance of a religion that you don't necessarily follow every single time you write the date: "AD" = "Anno Domini" = "year of Our Lord" (so writing AD is equivalent to saying "Jesus is my lord") and BC = "Before Christ" = "Before the Messiah" (so writing BC is equivalent to saying "Jesus is the Messiah"), and non-Christians occasionally get upset with the idea of needing to say what amounts to a small Christian prayer just to communicate simple concepts. Granted, this is more of a social statement regarding (and rejecting) the dominance of religion and religious iconography in general and/or this one in particular in language and thus culture than a scientific statement, but a statement nonetheless.
- Also popular with religious non-Christians using Western dating for comprehensibility reasons for similar but more doctrinal reasons.
The Tranquility calendar, first postulated in Omni magazine, is a 13-month revision of the Gregorian calendar, with the 1969 moon landing as its zero point (specifically, the exact moment Neil Armstrong said "tranquility" in "Houston, Tranquility Base here") and 28-day months named for scientists, plus days outside the months and weekday cycle, Armstrong Day (20 July Gregorian) and Aldrin Day (29 February Gregorian, only in leap years). Dates would have been either "B.T." (Before Tranquility) or "A.T." (After Tranquility). For example, 2013-05-17 corresponds to 21 Kepler, 44 A.T..
The French Enlightenment gave us August Comte's Positivist calendar, which named every day and month after one of history's great men. The new regime created after the French Revolution adopted the same kind of calendar based on humanistic values and working according to a simple mathematical system.
- After her Revolution, France tried a metric calendar. It wasn't very popular, for various reasons (ten-day weeks meant weekends were a lot rarer!), and only lasted twelve years. The names of the months and days of the week were changed, one of which (Thermidor, which lasted from July to August) is still remembered in the name of the dish Lobster Thermidor. They also had separate a name for each day of the year, trying to mimic the calendar of saints with something secular — in this case, plants, animals and tools. Which gave us day names like "pig" (Frimaire 5) or "manure" (Nivose 8) - yes, really, they had this. Now imagine all the teasing in school for kids born on these days...
- The above French calendars were parodied to an extent by the Pataphysical Calendar, the calendar created by the Society of Pataphysics. It is a calendar of 13 months of 29 days (the last of which has no weekday and is imaginary most of the time anyway), which begins on the date of birth of the founder of 'Pataphysics, and which has among other things days named for Don Quixote and Hari Seldon.
Soviets attempted this as well in the early 30's.
- Which was even more short-lived that the French system, mainly because at the point even authorities doubted whether that was really necessary.
Fascist Italy too had its very own calendar, in which 1922 marked the year zero; therefore, the year 1936 would have been XIV E.F. (that is, the fourteenth year of the "Fascist Era"). Remnants of this calendar can still be seen on the fa�ades of public buildings throughout Italy.
There have also been attempts to create calendars for other planets (and moons) in the solar system in case we need to live there someday. The most developed is the Darian calendar for Mars, which has 24 months of 28 Martian days each (called sols, so we don't confuse them with Earth days). There's already been arguments on what epoch to use (i.e. when Year Zero was).
- One would think that would be better decided when we actually get there.
- Just make sure there is a Year Zero, so we don't have the stupid "New century starts in —00/—01" argument again.
Calendars (by the same guy that did the Darian calendar for Mars) have also been proposed for the Jovian moons:
- Io: 24-month year, with 32 "circads" per month, an 8-circad week, 40 circads in the twelfth month (and the 24th month in leap years).
- Europa: 24-month year, 8-circad week, with 32 circads per month, plus a week in month 24 in leap years.
- Ganymede: 24-month year, 8-circad week, with 32 circads per month, minus one week in month 24 in non-leap years.
- Callisto: See Io.
The Japanese Era system, since 1872, has been based off of the lives of their emperors, beginning at the moment of the emperor's accession and ending with the incumbent's death. This makes eras fairly short, and if you're not versed in Japanese time systems, causes misunderstandings.
- Before the Meiji Restoration, eras could be changed at will, some lasting only a year or less. They were usually changed after a major event (sometimes to hopefully throw off the taint of whatever had occurred in the previous era).
- The last time when an emperor died (Hirohito, 1989), the Japanese computer programmers had to change the value for "era" from "Showa" to "Heisei" everywhere. Just like the Y2K bug.
Religions sometimes use different calendars to calculate holidays:
- Many Orthodox Christian churches still follow the Julian calendar, which is thirteen days different from the Gregorian, with the difference growing at a rate of about 3 days every 4 centuries.
  - They are slowly switching to the Gregorian calendar though.
- Judaism follows a calendar that is both solar and lunar, built around the New Moon as the start of the month, and counts years from the Biblical date of the creation of the universe^* (2008 corresponds to 5768/9 — the New Year is in September or thereabouts). The calendar uses a 19-year cycle, adding leap months in a set sequence of years, in order to keep all holidays falling in the same seasons.
- The Muslim (or Hijri) calendar has 12 lunar months in a year of about 354 days. Because this lunar year is about 11 days shorter than the solar year, Islamic holy days, although celebrated on fixed dates in their own calendar, usually shift 11 days earlier each successive solar year, such as a year of the Gregorian calendar. Islamic years are also called Hijra years because the first year was the year during which the Hijra occurred - Islamic prophet Muhammad's emigration from Mecca to Medina. Thus each numbered year is designated either H or AH, the latter being the initials of the Latin anno Hegirae (in the year of the Hijra). For example, AH 1433 is approximately 26 November 2011 (CE) to 15 November 2012.
  - The Islamic year 20874 AH will be the same year as 20874 AD.
- A rather peculiar one is the Iranian calendar. It can justifiably claim to be both one of the oldest and one of the newest calendars in wide use. Oldest, because the Iranians have had some sort of unique calendar for as long as they've been around; at least one of these remains the official calendar for Zoroastrian worship. Newest, because the calendar as it stands was invented in the 11th century (CE), and it combines the mathematical/calculating elements of the Hindu calendar (the most important of which is a highly complex system of calculating when to have the next leap year), month divisions based on the Chinese-Uighur calendar (the signs of the Zodiac), with the ancient Iranian calendar providing the date of the new year (the ancient spring festival of Nowruz, on the vernal equinox, during which men jump over bonfires and hope their crotches don't get set alight), and the Islamic epoch date (i.e. Year Zero is the year of the Hijra). As a result, the Iranian calendar is often called the Solar Hijri calendar. The Solar Hijri year for 2011-2012 is 1390 (note the 42-year difference between it and the Islamic, or Lunar Hijri year!).
- Discordians have a calendar that is divided into five 73-day seasons (Chaos, Discord, Confusion, Bureaucracy, Aftermath), with five-day weeks (Sweetmorn, Boomtime, Pungenday, Prickle-Prickle, Setting Orange), and two holidays a season, one on the fifth day and one on the fiftieth. The zero-point of the Discordian calendar is 1166 BCE, so 2008 AD would be 3174 Year of our Lady of Discord. Every fourth year includes a leapday analogue called St. Tib's Day, which has caused some unintended mathematical confusion as to whether the Discordian Calendar is meant to align with the Gregorian or Julian calendar.
Those Wacky Nazis tried to replace the Latin-derived months' names by more "Aryan" ones. This was the brain-child of Georg von Sch�nerer (a late 19th-century anti-semitic and far-right radical from Austria), and is from long before the Nazis' time.
In the Finnish language, every month really is called "______ Moon". For example, December is "Joulukuu", "Christmas moon".
- The same is true in Chinese. first-moon, second-moon, third-moon, etc. (一月,二月,三月...) Days of the week are handled similarly: 星期/周一,周二,周三.
  - As well as in Japanese, although many day names, despite being written in a regular way with the characters for the number and "day", are pronounced very irregularly: the first day of the month is tsuitachi, the second futsuka, the third mikka, the fourth yokka, the fifth itsuka, and so on.
  - The only one of those that's really irregular is tsuitachi - futsuka, mikka, yokka, itsuka, and so on until juuichi (the eleventh) is based off of native Japanese numbers: hi, fu, mi, yo, itsu, mu, nana, ya (although this one is youka), and tou.
  - Japanese also has traditional month names, like 神無月 'Kannazuki' (godless moon), 文月 'Fumizuki' (letter moon), and so on. Three of them don't end in -zuki (moon), 弥生 'Yayoi', 師走 'Shiwasu', and 如月 'Kisaragi' (although the last is written with 'moon').
Some Christian congregations have been known use terms like "First Day", "Second Day", and so on, rather than names like "Thursday" and "Saturday" that have pagan origins (honoring the Norse god Thor and the Roman god Saturn, respectively). They have been known to give the same treatment to the names of the months, to avoid naming Roman gods or emperors.
And not just congregations, but whole Christian cultural spheres, such as Portuguese and Estonian. They have numbered week days, and Saturday is either "sabado" (Sabbath) or "lauap�ev" (cleansing day) and Sunday is "domingo" and "p�hapaev" (Lord's Day) respectively.
- In Modern Hebrew the days never had names but always had numbers. They're just called "Day First", "Day Second", "Day Third" and so on until the seventh day, which has the name "Shabbat" or "Sabbath". And this is the case not only in modern, but also in medieval, Mishnaic and Biblical Hebrew, inasmuch as we see days of the week named.
  - The custom is common among Semitic languages; the Arabic days of the week (Al-Ahad, al-Ithnayn, al-Thulatha'...) mostly translate to "First, Second, Third...," but Friday is "al-Jumu`ah," or "Gathering" (because it's the day of praying together in the mosque) and Saturday is "al-Sabt". Al-Sabt comes from the same common the same root as "Shabbat," which appears to be influenced by (but not directly derived from) the common Semitic root S-B-` (realized in Arabic as saba`ah and in Hebrew as shev`a), as both Hebrew and Arabic reckon Sunday as the "first day" of the week. Al-Sabt is mentioned in The Quran as the day on which God rested after creating the world (as in The Book Of Genesis), and the Muslim Sabbath was on Saturday (and Muslims prayed toward Jerusalem) until the Jews of Medina pissed Muhammad and the Muslim community off. At that point, they changed the direction of prayer to Makkah (an almost literal about-face—Medina is about halfway between the two) and changed the Sabbath to Friday (honoring the last day of creation, on which He created Man), which became al-Jumu`ah to reflect this change (to pagan Arabs, Friday was al-Suds, "Sixth Day", while Saturday was always al-Sabt, reflecting common Semitic tradition). Interestingly, this allows the days of the week to line up with the workweek in some countries; some Arabic-speaking countries (e.g. Egypt) have Friday-Saturday weekends, meaning that the workweek starts on "First Day."
  - Sunday, Monday, Third Day sounds silly - to English speakers. Nevertheless, that's what happens in Slavic languages, such as Polish, which goes Niedziela (Rest), Poniedzialek (After Rest), Wtorek (Tuesday - second), Sroda (Wednesday - Middle), Czwartek (Thursday - fourth), Piatek (Friday - fifth) and Sobota (Sabbath). Russian works similarly, with the substitution of Voskreseniye - Resurrection - for Sunday. The two modern Baltic languages, Latvian and Lithuanian, have however ditched the non-numeral names and just number the week Firstday to Seventhday from Monday to Sunday respectively.
Armenians (traditionally) use a calendar based on the Ancient Egyptian one, with twelve 30-day months (and five monthless days) each day of the month having its own name, starting in 552 AD.
Interestingly, the Egyptian one is still used by the Coptic Orthodox Church as the Coptic calendar. Augustus Caesar modified it to be in sync with the Julian calendar, but that's about all that's changed since the days of the Pharaohs.
- Most modern Egyptians—both members of the Coptic Christian minority and Muslim majority—are barely aware of this calendar's existence, using the Gregorian calendar for almost everything. The Coptic calendar and Islamic lunar calendar are mostly only considered when it comes to the timing of religious festivals (plus one secular festival linked to a religious one). However, the Gregorian calendar is a fairly recent introduction, being instituted sometime in the 19th century (probably by the Khedive Ismail, who tried to be more European than the Europeans themselves). Before that time, Egyptians Muslim and Christian alike used the Coptic calendar for agricultural purposes: the months are coordinated with the significant events of Egypt's unique agricultural situation, including the floods of the Nile and the rains (which are so regular that you can predict almost to the day what the weather in Alexandria will be like a whole year in advance).
- Another interesting tidbit: the Coptic Church uses as its epoch the accession of the Roman Emperor Diocletian (in 284), noted for his persecutions of Christians, which by that time was very strong in Egypt. As a result, the Coptic calendar is often marked with the abbreviation A.M.—Anno Martyrum (Year of the Martyrs).
There are dozens of calendar systems in place, including the Hindu calendar, the Zoroastrian calendar, the Ethiopic calendar, and several other local ones. Just look it up on Wikipedia.
Although computers generally use the Gregorian calendar to display dates to the user, internally, most software represents time as an integer number: the number of seconds or milliseconds since 1 January 1970, the so-called UNIX epoch. This has in turn led to speculation about a possible Y2K-like phenomenon where the UNIX time varible overflows, which could happen around year 2038 if a signed 32-bit integer is used to represent seconds. Modern systems are just now becoming 64-bit, which'll last for about another 300 billion years. Whether all software gets converted over to 64bit before 2038 is another issue, however.
- However, since the UNIX timestamp second count is kept in sync with UTC, which uses occasional leap seconds to keep it consistent within a second of Greenwich Mean Time, the numeric value of this count is not exactly equal to the actual number of elapsed seconds, because the count is defined so that there are always 86,400 seconds in a day, so any leap seconds are skipped in the count.
In Basic programming, dates are stored as a double-length decimal number representing the number of days from 1/1/1900. This is why in Excel or Access, to add a day, you simply do "Date+1". This does not easily take into account daylight savings time, as non-existent times can be shown during the skip forward, and there is no distinction between the first and second version of 2-3AM during the skip back.
Most natives of the Arctic Circle have traditionally described time's passage in terms of "sleeps", as days' actual duration varies too widely at high latitudes to be a reliable measurement.
Infamously, Pol Pot's 'Democratic Kampuchea' created a "Year Zero" during his genocidal rule of Cambodia
Historians writing about Rome (both at the time and more recently) sometimes use AUC, Ab Urbe Condita="from the founding of the city (of Rome)", dating years from the traditional founding of Rome in 753 BC. This system has been copied by various writers with different cities. However, Romans usually reckoned years by who was emperor. In the Republican era and early empire, years were named for the two consuls, which may be just as well, as nobody knows exactly when Rome was founded.
- Which makes the Gregorian Calander a meta-example of this. The average Roman tended to recon their time with the Emperors, and since Christ is the King of Kings...
Turkmenistan's rubber-stamp government decided to rename their entire calendar, INCLUDING the days of the week, in 2002. This was a basic decree by their somewhat insane leader, Saparmurat Niyazov, aka the Turkmenbashi and one-time administrative assistant to the Mayor of Cloudcuckooland. Since his death, such decrees have been removed from the book of law.
The North Korean calendar begins with the birth of Kim Il Sung (1912 AD) as year Juche 1 in Gregorian format; all pre-calendar history uses the Gregorian calendar.
By a stunning coincidence, the Chinese system of reckoning the year by the years of the reign of the Emperor has led to the Taiwanese calendar being precisely in sync with the Juche one: the Republic of China (aka Taiwan) elected to continue the era-name system, using "Minguo" (Republic) as the name. Since the republic was founded in 1912, each Minguo year has the same number as the corresponding Juche year (2010, for instance, being both Minguo 99—the 99th Year of the Republic—and Juche 99). This has led to the somewhat amusing Y1C Problem for Taiwanese computer systems (which use the Minguo year).
In Ancient Greece, every polis had its own calendar, with years named after government officials (like ephors in Sparta or archons in Athens). These calendars were very often very quirky (for instance, Athens had a calendar of ten months, which had some interesting connections to the Athenian constitution).*
If you must insist, the citizens of Athens were divided into ten tribes. Every year, 50 male members of each tribe were selected—by lot!—to sit on the "Council of 500", aka the Boule. The Boule set the agenda of the Ekklesia, or Popular Assembly (the assembly of all free male citizens of Athens, which had final authority on everything), and certain other responsibilities as well. Of these, the most important of was that of sitting vigil. Each of the ten Athenian tribes was assigned a month. During that month, its 50 members of the Boule were expected to remain at the Boule's building day and night in order to respond quickly to any crisis that might arise. Convenient, isn't it, that there were ten months and ten tribes?
Must have been a headache with over a thousand different states in place. The historians sometimes used the Olympic games for counting the years, but no one else did.
Javanese people in Indonesia uses month names taken from Islamic calendar. The month names are Sura (Muharram), Sapar (Safar), Mulud (Rabi al-awwal), Bakda Mulud (Rabi al-thani), Jumadil Awal (Jumada al-awwal), Jumadil Akhir (Jumada al-thani), Rejeb (Rajab), Ruwah (Sha'aban), Pasa (Ramadhan), Sawal (Shawwal), Sela (Dhul al-Qi'dah) and Besar (Dhul al-Hijjah).
In Starhawk's novel The Fifth Sacred Thing, the largely neo-Pagan community in a future San Francisco refers to August as "Third Foggy Moon". (This is drawn from a San Francisco calendar Starhawk proposed in her nonfiction book Truth or Dare: as a substitute for the "traditional" neo-Pagan months, she worked a set of month-names appropriate to the place and time where she was working. The three full moons of summer are "Fog Rolls In Moon", "Fog Sticks Around Moon", and "Fog Sticks Around Some More Moon".)