# Year zero

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Year used in some calendars

For other uses, see [Year zero (disambiguation)](/source/Year_zero_(disambiguation)). "0 A.D." redirects here. For the video game, see [*0 A.D.* (video game)](/source/0_A.D._(video_game)).

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A **year zero** is a date where the current year for a given calendar system is zero. In systems which include a year zero, this year would be the [epoch](/source/Epoch). Year zero does not exist in the [Anno Domini](/source/Anno_Domini) (AD) calendar year system commonly used to number years in the [Gregorian calendar](/source/Gregorian_calendar) and [Julian calendar](/source/Julian_calendar). Instead, [AD 1](/source/0s) is treated as the epoch, so that the year [1 BC](/source/1_BC) is followed directly by year [AD 1](/source/AD_1). However, there is a year zero in both the [astronomical year numbering](/source/Astronomical_year_numbering) system (where it coincides with the Julian year 1 BC), and the [ISO 8601:2004](/source/ISO_8601) system, a data interchange standard for certain time and calendar information (where year zero coincides with the Gregorian year 1 BC; see: [Holocene calendar § Conversion](/source/Holocene_calendar#Conversion)). There is also a year zero in most [Buddhist](/source/Buddhist_calendar) and [Hindu calendars](/source/Hindu_calendar).

## History

The *[Anno Domini](/source/Anno_Domini)* era was introduced in 525 by [Scythian](/source/Scythia_Minor) monk [Dionysius Exiguus](/source/Dionysius_Exiguus) (c. 470 – c. 544), who used it to identify the years on his [Easter table](/source/Dionysius_Exiguus'_Easter_table). He introduced the new era to avoid using the *[Diocletian era](/source/Diocletian_Era)*, based on the accession of [Roman emperor](/source/Roman_emperor) [Diocletian](/source/Diocletian), as he did not wish to continue the memory of [a persecutor of Christians](/source/Diocletianic_Persecution). In the preface to his Easter table, Dionysius stated that the "present year" was "the [consulship](/source/Consulship) of [Probus Junior](/source/Probus_(consul_525))" which was also 525 years "since the incarnation of our Lord [Jesus Christ](/source/Jesus_Christ)".[1] How he arrived at that number is unknown.[2]

Dionysius Exiguus did not use "AD" years to date any historical event. This practice began with the English cleric [Bede](/source/Bede) (c. 672–735), who used AD years in his *[Historia ecclesiastica gentis Anglorum](/source/Historia_ecclesiastica_gentis_Anglorum)* (731), popularizing the era. Bede also used – only once – a term similar to the modern English term "[before Christ](/source/Anno_Domini)", though the practice did not catch on for nearly a thousand years, when books by [Denis Pétau](/source/Denis_P%C3%A9tau) treating calendar science gained popularity. Bede did not sequentially number [days of the month](/source/Roman_calendar#Months), weeks of the year, or months of the year. However, he did number many of the [days of the week](/source/Week#Days_of_the_week) using the counting origin one in [Ecclesiastical Latin](/source/Ecclesiastical_Latin).[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]

Previous Christian histories used several titles for dating events: *[anno mundi](/source/Anno_mundi)* ("in the year of the world") beginning on the purported first day of [creation](/source/Creation_myth); or *anno Adami* ("in the year of [Adam](/source/Adam)") beginning at the creation of Adam five days later (or the sixth day of creation according to the [Genesis creation narrative](/source/Genesis_creation_narrative)) as used by [Africanus](/source/Sextus_Julius_Africanus); or *anno Abrahami* ("in the year of [Abraham](/source/Abraham)") beginning 3,412 years after Creation according to the [Septuagint](/source/Septuagint), used by [Eusebius of Caesarea](/source/Eusebius_of_Caesarea); all of which assigned "one" to the year beginning at Creation, or the creation of Adam, or the birth of Abraham, respectively. Bede continued this earlier tradition relative to the AD era.[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]

In chapter II of book I of *Ecclesiastical History*, Bede stated that [Julius Caesar](/source/Julius_Caesar) invaded Britain "in the year 693 after the building of Rome, but the 60th year before the incarnation of our Lord", while stating in chapter III, "in the year of Rome 798, [Claudius](/source/Claudius)" also invaded Britain and "within a very few days ... concluded the war in ... the 46th [year] from the incarnation of our Lord".[3] Although both dates are wrong, they are sufficient to conclude that Bede did not include a year zero between BC and AD: 798 − 693 + 1 (because the years are inclusive) = 106, but 60 + 46 = 106, which leaves no room for a year zero. The modern English term "before Christ" (BC) is only a rough equivalent, *not* a direct translation, of Bede's Latin phrase *ante incarnationis dominicae tempus* ("before the time of the lord's incarnation"), which was itself never abbreviated. Bede's singular use of 'BC' continued to be used sporadically throughout the [Middle Ages](/source/Middle_Ages).[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]

Neither the concept of nor a symbol for zero existed in the system of [Roman numerals](/source/Roman_numerals). The [Babylonian system](/source/Babylonian_cuneiform_numerals) of the BC era had used the idea of "nothingness" without considering it a number, and the Romans enumerated in much the same way. Wherever a modern zero would have been used, Bede and Dionysius Exiguus did use [Latin](/source/Latin) number words, or the word *nulla* (meaning "nothing") alongside Roman numerals.[1][4][5] Zero was invented in India in the sixth century,[6] and was either transferred or reinvented by the Arabs by about the eighth century. The [Arabic numeral](/source/Arabic_numerals) for zero ([0](/source/0)) did not enter Europe until the 13th century. Even then, it was known only to very few, and only entered widespread use in Europe by the 17th century.[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]

The *anno Domini* nomenclature was not widely used in [Western Europe](/source/Western_Europe) until the 9th century, and the 1 January to 31 December historical year was not uniform throughout Western Europe until 1752. The first extensive use (hundreds of times) of 'BC' occurred in *Fasciculus Temporum* by [Werner Rolevinck](/source/Werner_Rolevinck) in 1474, alongside years of the world (*anno mundi*).[7] The terms *anno Domini*, *Dionysian era*, *Christian era*, *vulgar era*, and *common era* were used interchangeably between the [Renaissance](/source/Renaissance) and the 19th century, at least in [Latin](/source/Latin). But *vulgar era* fell out of use in English at the beginning of the 20th century after *vulgar* acquired the meaning of "offensively coarse", replacing its original meaning of "common" or "ordinary". Consequently, historians regard all these eras as equal.[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]

Historians have never included a year zero. This means that between, for example, 1 January 500 BC and 1 January AD 500, there are 999 years: 500 years BC, and 499 years AD preceding 500. In common usage *anno Domini* 1 is preceded by the year 1 [BC](/source/Ante_Christum_natum), without an intervening year zero.[8] Neither the choice of calendar system (whether [Julian](/source/Julian_calendar) or [Gregorian](/source/Gregorian_calendar)) nor the name of the era (*Anno Domini* or [Common Era](/source/Common_Era)) determines whether a year zero will be used. If writers do not use the convention of their group (historians or astronomers), they must explicitly state whether they include a year 0 in their count of years, otherwise their historical dates will be misunderstood.[9]

## Astronomy

Main article: [Astronomical year numbering](/source/Astronomical_year_numbering)

In astronomy, for the year AD 1 and later it is common to assign the same numbers as the Anno Domini notation, which in turn is numerically equivalent to the Common Era notation. But the discontinuity between 1 AD and 1 BC makes it cumbersome to compare ancient and modern dates. So the year before 1 AD is designated 0, the year before 0 is −1, and so on.

The letters "AD", "BC", "CE", or "BCE" are omitted. So 1 BC in historical notation is equivalent to 0 in astronomical notation, 2 BC is equivalent to −1, etc. Sometimes positive years are preceded by the + sign. This year numbering notation was introduced by the astronomer [Jacques Cassini](/source/Jacques_Cassini) in 1740.[10]

### History of astronomical usage

In 1627, the German astronomer [Johannes Kepler](/source/Johannes_Kepler), in his *[Rudolphine Tables](/source/Rudolphine_Tables)*, first used an astronomical year essentially as a year zero. He labeled it *Christi* and inserted it between years labeled *Ante Christum* and *Post Christum*—abbreviated BC and AD today, respectively—on the "mean motion" pages of the Sun, Moon, and planets.[11] In 1702, the French astronomer [Philippe de La Hire](/source/Philippe_de_La_Hire) labeled a year as *Christum* *0* and placed it at the end of the years labeled *ante Christum* (BC), and immediately before the years labeled *post Christum* (AD), on the mean motion pages in his *Tabulæ Astronomicæ*, thus adding the number designation *0* to Kepler's *Christi*.[12]

Finally, in 1740, the transition was completed by French astronomer [Jacques Cassini](/source/Jacques_Cassini) (Cassini II), who is traditionally credited with inventing year zero.[13] In his *Tables astronomiques*, Cassini labeled the year simply as *0*, and placed it at the end of years labeled *avant Jesus-Christ* (BC), and immediately before years labeled *après Jesus-Christ* (AD).[14]

## ISO 8601

[ISO 8601:2004](/source/ISO_8601) (and previously ISO 8601:2000, but not ISO 8601:1988) explicitly uses astronomical year numbering in its date reference systems. Because it also specifies the use of the [proleptic Gregorian calendar](/source/Proleptic_Gregorian_calendar) for all years before 1582, some readers incorrectly assume that a year zero is also included in that proleptic calendar, but it is not used with the BC/AD era. The "basic" format for year 0 is the four-digit form 0000, which equals the historical year 1 BC. Several "expanded" formats are possible: −0000 and +0000, as well as five- and six-digit versions. Earlier years are also negative four-, five- or six-digit years, which have an [absolute value](/source/Absolute_value) one less than the equivalent BC year, hence -0001 = 2 BC. Because only [ISO 646](/source/ISO_646) (7-bit [ASCII](/source/ASCII)) characters are allowed by ISO 8601, the minus sign is represented by a [hyphen-minus](/source/Hyphen-minus).

## Computing

Programming libraries may implement a year zero, an example being the [Perl](/source/Perl) [CPAN](/source/CPAN) module DateTime.[15]

## Indian calendars

This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Year zero" – news · newspapers · books · scholar · JSTOR (December 2023) (Learn how and when to remove this message)

Most eras used with [Hindu](/source/Hindu_calendar) and [Buddhist calendars](/source/Buddhist_calendar), such as the [Saka era](/source/Saka_era) or the [Kali Yuga](/source/Kali_Yuga), begin with the year 0.[16] These calendars mostly use elapsed, expired, or complete years, in contrast with most calendars from other parts of the world which use current years. A complete year had not yet elapsed for any date in the initial year of the epoch, thus the number 1 cannot be used. Instead, during the first year the indication of 0 years (elapsed) is given in order to show that the epoch is less than 1 year old. This is similar to the Western method of stating a person's age – people do not reach age one until one year has elapsed since birth (but their age during the year beginning at birth is specified in months or fractional years, not as age zero). However, if ages were specified in years and months, such a person would be said to be, for example, 0 years and 6 months or 0.5 years old. This is analogous to the way time is shown on a [24-hour clock](/source/24-hour_clock): during the first hour of a day, the time elapsed is 0 hours, *n* minutes.

## See also

- [List of non-standard dates](/source/List_of_non-standard_dates)

- [Time formatting and storage bugs](/source/Time_formatting_and_storage_bugs)

- [Zero-based numbering](/source/Zero-based_numbering)

- [Off-by-one error](/source/Off-by-one_error)

## References

1. ^ [***a***](#cite_ref-Dionysius_1-0) [***b***](#cite_ref-Dionysius_1-1) ["Dionysius Exiguus. Liber de paschate sive cyclus paschalis"](https://web.archive.org/web/20060109025617/http://hbar.phys.msu.su/gorm/chrono/paschata.htm). Archived from [the original](http://hbar.phys.msu.su/gorm/chrono/paschata.htm) on 9 January 2006.

1. **[^](#cite_ref-2)** Blackburn, Bonnie; Holford-Strevens, Leofranc (2003). *The Oxford Companion to the Year: An exploration of calendar customs and time-reckoning* (corrected reprinting of 1999 ed.). Oxford University Press. pp. 778–779. [ISBN](/source/ISBN_(identifier)) [9780192142313](https://en.wikipedia.org/wiki/Special:BookSources/9780192142313).

1. **[^](#cite_ref-3)** ["Ecclesiastical History of the English Nation"](http://www.fordham.edu/halsall/basis/bede-book1.html). [Archived](https://web.archive.org/web/20090830014333/http://www.fordham.edu/halsall/basis/bede-book1.html) from the original on 30 August 2009. Retrieved 5 August 2005.

1. **[^](#cite_ref-4)** Faith Wallis, trans. *Bede: The Reckoning of Time* (725), Liverpool: Liverpool University Press, 2004. [ISBN](/source/ISBN_(identifier)) [0-85323-693-3](https://en.wikipedia.org/wiki/Special:BookSources/0-85323-693-3).

1. **[^](#cite_ref-5)** *Byrhtferth's Enchiridion* (1016). Edited by Peter S. Baker and Michael Lapidge. Early English Text Society 1995. [ISBN](/source/ISBN_(identifier)) [978-0-19-722416-8](https://en.wikipedia.org/wiki/Special:BookSources/978-0-19-722416-8).

1. **[^](#cite_ref-6)** [One of three civilizations to invent zero](https://www.scientificamerican.com/article/what-is-the-origin-of-zer/#:~:text=The%20first%20recorded%20zero%20appeared,the%20end%20of%20the%20eighth.))

1. **[^](#cite_ref-7)** Werner Rolevinck, *[Fasciculus temporum](http://www.cervantesvirtual.com/servlet/SirveObras/80248064097793506388868/index.htm) [Archived](https://web.archive.org/web/20160210203337/http://www.cervantesvirtual.com/servlet/SirveObras/80248064097793506388868/index.htm) 10 February 2016 at the [Wayback Machine](/source/Wayback_Machine)*.

1. **[^](#cite_ref-8)** While it is increasingly common to place *AD* after a date by analogy to the use of *BC*, formal English usage adheres to the traditional practice of placing the abbreviation before the year as in Latin (e.g., 100 BC, but AD 100).

1. **[^](#cite_ref-9)** V. Grumel, *La chronologie* (1958), page 30.

1. **[^](#cite_ref-10)** Richards, E. G. (2013). "Calendars". In Urban, Sean E.; Seidelmann, P. Kenneth (eds.). *Explanatory Supplement to the Astronomical Almanac* (3 ed.). Mill Valley, California: University Science Books. p. 591. [ISBN](/source/ISBN_(identifier)) [978-1-891389-85-6](https://en.wikipedia.org/wiki/Special:BookSources/978-1-891389-85-6).

1. **[^](#cite_ref-11)** ["Tabulae Rudolphinae – Ioannes Keplerus"](http://www.ub.uni-kiel.de/digiport/bis1800/Arch3_436.html) [Archived](https://web.archive.org/web/20170511095530/http://www.ub.uni-kiel.de/digiport/bis1800/Arch3_436.html) 11 May 2017 at the [Wayback Machine](/source/Wayback_Machine) (1627) 191 (42), 197 (48), 203 (54), 209 (60), 215 (66), 221 (72), 227 (78).

1. **[^](#cite_ref-12)** [Tabulae Astronomicae – Philippo de la Hire](https://books.google.com/books?id=BjygAAAAMAAJ) [Archived](https://web.archive.org/web/20230326164823/https://books.google.com/books?id=BjygAAAAMAAJ) 26 March 2023 at the [Wayback Machine](/source/Wayback_Machine) (1702), Tabulæ 15, 21, 39, 47, 55, 63, 71; Usus tabularum 4.

1. **[^](#cite_ref-13)** Kaplan, Robert (28 October 1999). [*The Nothing that Is: A Natural History of Zero*](https://books.google.com/books?id=Bn0EBVsfi1YC&pg=PA103). Oxford University Press. [ISBN](/source/ISBN_(identifier)) [978-0-19-802945-8](https://en.wikipedia.org/wiki/Special:BookSources/978-0-19-802945-8). [Archived](https://web.archive.org/web/20230630062141/https://books.google.com/books?id=Bn0EBVsfi1YC&pg=PA103) from the original on 30 June 2023.

1. **[^](#cite_ref-14)** [Jacques] Cassini, *Tables astronomiques* (1740), Explication et usage 5; Tables 10, 22, 53.

1. **[^](#cite_ref-15)** ["DateTime – A date and time object – metacpan.org"](https://metacpan.org/pod/release/DROLSKY/DateTime-1.03/lib/DateTime.pm#Floating-DateTimes). [Archived](https://web.archive.org/web/20200305100057/https://metacpan.org/pod/release/DROLSKY/DateTime-1.03/lib/DateTime.pm#Floating-DateTimes) from the original on 5 March 2020. Retrieved 2 April 2019.

1. **[^](#cite_ref-16)** ["Hindu Eras and Epochs"](https://www.learnreligions.com/hindu-months-days-eras-and-epochs-1770056). Retrieved 1 September 2025.

v t e Chronology Key topics Archaeology Astronomy Geology History Big History Paleontology Time Periods Eras Epochs Calendar eras Human Era Ab urbe condita Anno Domini / Common Era Anno Mundi Bosporan era Bostran era Byzantine era Seleucid era Era of Caesar (Iberia) Before Present Hijri Egyptian Sothic cycle Hindu units of time (Yuga) Jain units of time (Avasarpiṇī) Mesoamerican Long Count Short Count Tzolkʼin Haabʼ Regnal year Anka year Canon of Kings English and British regnal year Lists of kings Limmu Era names Chinese Japanese Korean Vietnamese Calendars Pre-Julian / Julian Pre-Julian Roman Original Julian Proleptic Julian Revised Julian Gregorian Gregorian Proleptic Gregorian Old Style and New Style dates Adoption of the Gregorian calendar Dual dating Astronomical Lunisolar (Hebrew, Hindu) Solar Lunar (Islamic) Astronomical year numbering Others Chinese sexagenary cycle Geologic Calendar Iranian ISO week date Mesoamerican Maya Aztec Winter count New Earth Time Astronomic time Cosmic Calendar Ephemeris Galactic year Metonic cycle Milankovitch cycles Geologic time Concepts Deep time Geological history of Earth Geological time units Standards Global Standard Stratigraphic Age (GSSA) Global Boundary Stratotype Section and Point (GSSP) Methods Chronostratigraphy Geochronology Isotope geochemistry Law of superposition Luminescence dating Samarium–neodymium dating Chronological dating Absolute dating Amino acid racemisation Archaeomagnetic dating Dendrochronology Ice core Incremental dating Lichenometry Paleomagnetism Radiometric dating Lead–lead Potassium–argon Radiocarbon Uranium–lead Tephrochronology Luminescence dating Thermoluminescence dating Relative dating Fluorine absorption Nitrogen dating Obsidian hydration Seriation Stratigraphy Genetic methods Molecular clock Linguistic methods Glottochronology Related topics Chronicle New Chronology Synchronoptic view Timeline Year zero Floruit Terminus post quem ASPRO chronology

v t e Calendars Systems Lunar Lunisolar Solar In wide use Bengali Buddhist Chinese Ethiopian Gregorian Hebrew Hindu Islamic Śaka Solar Hijri In limited use Akan Armenian Assamese Assyrian Baháʼí Bali Pawukon Saka Bangladeshi Berber Borana Burmese Germanic heathen Georgian Hmong Igbo Iranian Jalali Zoroastrian Irish Islamic Fasli Tabular Jain Japanese Javanese Korean Kurdish Lithuanian Maithili Malayalam Mandaean Manipuri (Meitei) Melanau Minguo Mizo (Lushai) Mongolian Nepali Nepal Sambat Yele Sambat Nanakshahi Nisg̱a'a Odia Punjabi Romanian Shona Somali Sesotho Slavic Slavic Native Faith Macedonian Sundanese Tamil Thai lunar solar Tibetan Tripuri Tulu Vietnamese Vikram Samvat Wicca Xhosa Yoruba Zulu Types Runic Mesoamerican Long Count Calendar round Christian variants Coptic Ethiopian and Eritrean Julian Revised Liturgical year Eastern Orthodox Saints Historical Arabian Attic Aztec Tōnalpōhualli Xiuhpōhualli Babylonian Bulgar Byzantine Cappadocian Celtic Cham Culāsakaraj Coligny Egyptian Enoch Florentine French Republican Germanic Greek Inca Juche Macedonian Maya Haabʼ Tzolkʼin Muisca Pentecontad Pisan Qumran Rapa Nui Roman Rumi Sexagenary Soviet Swedish Turkmen By specialty Holocene anthropological Astronomical Proleptic Gregorian / Proleptic Julian historiographical ISO week Darian Martian Dreamspell New Age Discordian 'Pataphysical Unix time Reform proposals Hanke–Henry Permanent International Fixed Pax Positivist Symmetry454 World Displays and applications Electronic Perpetual Wall Year naming and numbering Terminology Era Epoch Leap year New Year Regnal name Regnal year Year zero Systems Ab urbe condita Anka year Anno Domini/Common Era Anno Lucis Anno Martyrum Anno Mundi Assyrian Before Present Chinese Imperial English and British regnal year Hijri year Japanese Korean Nirayana system Samvatsara Seleucid Sexagenary Spanish Yugas Satya Treta Dvapara Kali Vietnamese Fictional Discworld (Discworld) Greyhawk (Dungeons & Dragons) Middle-earth (The Lord of the Rings) Stardate (Star Trek) List of calendars Category

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Adapted from the Wikipedia article [Year zero](https://en.wikipedia.org/wiki/Year_zero) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Year_zero?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.