Ancient dating systems
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 has been 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 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 for indicating the passage of years was based on the order of
Olympic Games, first held in
776 BC. The Olympic games provided the various independent city-states a mutually recognizable system of dates. Olympiad dating was not used in everyday life. This system was in use from the 3rd century. The modern Olympic Games (or Summer Olympic Games beginning 1896) do not continue the four year periods from ancient Greece; 669th Olympiad would have begun in the summer of 1897, but the modern Olympics were first held in 1896.
Another common system was the
indiction cycle (15 indictions made up an agricultural tax cycle, 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
Egypt, and in most parts of Greece 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.
 Thus 2001 was the ninth indiction.
 The beginning of the year varied.
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 January 2 of the relevant civil year. 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.
Dating from the founding of Rome
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 (April 21), although most modern historians assume that it coincides with the modern historical year (January 1 to December 31). 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 2017 is thus approximately the same as AUC 2770 (2017 + 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).
Regnal years of Roman emperors
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.
Dating from the Roman conquest
Some regions of the
Roman Empire dated their calendars from the date of Roman conquest, or the establishment of Roman rule.
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,
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 August 11, 3114 BC in the proleptic Gregorian calendar or 6 September in the Julian calendar (−3113 astronomical).
Other dating systems
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