|Ab urbe condita||2774|
|Balinese saka calendar||1942–1943|
|British Regnal year||69 Eliz. 2 – 70 Eliz. 2|
|Chinese calendar||庚子年 (Metal Rat)|
4717 or 4657
— to —
辛丑年 (Metal Ox)
4718 or 4658
|- Vikram Samvat||2077–2078|
|- Shaka Samvat||1942–1943|
|- Kali Yuga||5121–5122|
|Japanese calendar||Reiwa 3|
|Julian calendar||Gregorian minus 13 days|
|Minguo calendar||ROC 110|
|Thai solar calendar||2564|
2147 or 1766 or 994
— to —
2148 or 1767 or 995
|Unix time||1609459200 – 1640995199|
A calendar era is the period of time elapsed since one epoch of a calendar and, if it exists, before the next one. For example, it is the year 2021 as per the Gregorian calendar, which numbers its years in the Western Christian era (the Coptic Orthodox and Ethiopian Orthodox churches have their own Christian eras).
In antiquity, regnal years were counted from the accession of a monarch. This makes the chronology of the ancient Near East very difficult to reconstruct, based on disparate and scattered king lists, such as the Sumerian King List and the Babylonian Canon of Kings. In East Asia, reckoning by era names chosen by ruling monarchs ceased in the 20th century except for Japan, where they are still used.
For over a thousand years, ancient Assyria used a system of eponyms to identify each year. Each year at the Akitu festival (celebrating the Mesopotamian new year), one of a small group of high officials (including the king in later periods) would be chosen by lot to serve as the limmu for the year, which meant that he would preside over the Akitu festival and the year would bear his name. The earliest attested limmu eponyms are from the Assyrian trading colony at Karum Kanesh in Anatolia, dating to the very beginning of the 2nd millennium BC, and they continued in use until the end of the Neo-Assyrian Period, ca. 612 BC.
Assyrian scribes compiled limmu lists, including an unbroken sequence of almost 250 eponyms from the early 1st millennium BC. This is an invaluable chronological aid, because a solar eclipse was recorded as having taken place in the limmu of Bur-Sagale, governor of Guzana. Astronomers have identified this eclipse as one that took place on 15 June 763 BC, which has allowed absolute dates of 892 to 648 BC to be assigned to that sequence of eponyms. This list of absolute dates has allowed many of the events of the Neo-Assyrian Period to be dated to a specific year, avoiding the chronological debates that characterize earlier periods of Mesopotamian history.
Among the ancient Greek historians and scholars, a common method of indicating the passage of years was based on the Olympic Games, first held in 776 BC. The Olympic Games provided the various independent city-states with a mutually recognizable system of dates. Olympiad dating was not used in everyday life. This system was in use from the 3rd century BC. The modern Olympic Games (or Summer Olympic Games beginning 1896) do not continue the four year periods from ancient Greece: the 669th Olympiad would have begun in the summer of 1897, but the modern Olympics were first held in 1896.: 769
Another common system was the indiction cycle (15 indictions made up an agricultural tax cycle in Roman Egypt, an indiction being a year in duration). Documents and events began to be dated by the year of the cycle (e.g., "fifth indiction", "tenth indiction") in the 4th century, and this system was used long after the tax ceased to be collected. It was used in Gaul, in Egypt until the Islamic conquest, and in the Eastern Roman Empire until its conquest in 1453.
The rule for computing the indiction from the AD year number, which he had just invented, was stated by Dionysius Exiguus: add 3 and divide by 15; the remainder is the indiction, with 0 understood to be the fifteenth indiction.: 770 Thus 2001 was the ninth indiction. The beginning of the year for the indiction varied.: 769–71
The Seleucid era was used in much of the Middle East from the 4th century BC to the 6th century AD, and continued until the 10th century AD among Oriental Christians. The era is computed from the epoch 312 BC: in August of that year Seleucus I Nicator captured Babylon and began his reign over the Asian portions of Alexander the Great's empire. Thus depending on whether the calendar year is taken as starting on 1 Tishri or on 1 Nisan (respectively the start of the Jewish civil and ecclesiastical years) the Seleucid era begins either in 311 BC (the Jewish reckoning) or in 312 BC (the Greek reckoning: October–September).
An early and common practice was Roman 'consular' dating. This involved naming both consules ordinarii who had taken up this office on 1 January (since 153 BC) of the relevant civil year.: 6 Sometimes one or both consuls might not be appointed until November or December of the previous year, and news of the appointment may not have reached parts of the Roman empire for several months into the current year; thus we find the occasional inscription where the year is defined as "after the consulate" of a pair of consuls.
The use of consular dating ended in AD 541 when the emperor Justinian I discontinued appointing consuls. The last consul nominated was Anicius Faustus Albinus Basilius. Soon afterwards, imperial regnal dating was adopted in its place.
Another method of dating, rarely used, was anno urbis conditae (Latin: "in the year of the founded city" (abbreviated AUC), where "city" meant Rome). (It is often incorrectly given that AUC stands for ab urbe condita, which is the title of Titus Livius's history of Rome.)
Several epochs were in use by Roman historians. Modern historians usually adopt the epoch of Varro, which we place in 753 BC.
The system was introduced by Marcus Terentius Varro in the 1st century BC. The first day of its year was Founder's Day (21 April), although most modern historians assume that it coincides with the modern historical year (1 January to 31 December). It was rarely used in the Roman calendar and in the early Julian calendar – naming the two consuls that held office in a particular year was dominant. AD 2021 is thus approximately the same as AUC 2774 (2021 + 753).
About AD 400, the Iberian historian Orosius used the AUC era. Pope Boniface IV (about AD 600) may have been the first to use both the AUC era and the Anno Domini era (he put AD 607 = AUC 1360).
Another system that is less commonly found than might be thought was the use of the regnal year of the Roman emperor. At first, Augustus indicated the year of his reign by counting how many times he had held the office of consul, and how many times the Roman Senate had granted him Tribunican powers, carefully observing the fiction that his powers came from these offices granted to him, rather than from his own person or the many legions under his control. His successors followed his practice until the memory of the Roman Republic faded (about AD 200), when they began to use their regnal year openly.
Some regions of the Roman Empire dated their calendars from the date of Roman conquest, or the establishment of Roman rule.
The Spanish era counted the years from 38 BC, probably the date of a new tax imposed by the Roman Republic on the subdued population of Iberia. The date marked the establishment of Roman rule in Spain and was used in official documents in Portugal, Aragon, Valencia, and in Castile, into the 14th century. This system of calibrating years fell to disuse in 1381 and was replaced by today's Anno Domini.
Throughout the Roman and Byzantine periods, the Decapolis and other Hellenized cities of Syria and Palestine used the Pompeian era, counting dates from the Roman general Pompey's conquest of the region in 63 BC.
A different form of calendar was used to track longer periods of time, and for the inscription of calendar dates (i.e., identifying when one event occurred in relation to others). This form, known as the Long Count, is based upon the number of elapsed days since a mythological starting-point. According to the calibration between the Long Count and Western calendars accepted by the great majority of Maya researchers (known as the GMT correlation), this starting-point is equivalent to 11 August, 3114 BC in the proleptic Gregorian calendar or 6 September in the Julian calendar (−3113 astronomical).
A great many local systems or eras were also important, for example the year from the foundation of one particular city, the regnal year of the neighboring Persian emperor, and eventually even the year of the reigning Caliph.
The era based on the Incarnation of Christ was introduced by Dionysius Exiguus in 525 and is in continued use with various reforms and derivations. The distinction between the Incarnation being the conception or the Nativity of Jesus was not drawn until the late ninth century.: 881 The beginning of the numbered year varied from place to place: when, in 1600, Scotland adopted 1 January as the date the year number changes, this was already the case in much of continental Europe. England adopted this practice in 1752.: 7
The Hindu Saka Era influences the calendars of southeast Asian indianized kingdoms.