A calendar era refers to the span of time that has passed since a specific point in a calendar and up until the next one, if applicable. As an example, the Gregorian calendar is currently in the year 2023, which is counted in the Western Christian era. However, other Christian denominations, such as the Coptic Orthodox and Ethiopian Orthodox, have their own unique Christian eras.

In the past, years were often recorded based on the reign of a particular monarch. This method of counting time makes it challenging to accurately chronicle the history of ancient civilizations in the Near East, as information is often fragmented and found in disparate sources such as the Sumerian King List and the Babylonian Canon of Kings. In the 20th century, the practice of recording time based on the era name selected by ruling monarchs ceased in most of East Asia, with the exception of Japan where it is still utilized.

Evolution of Calender Era

Ancient Period

Assyrian Eponyms

• Ancient Assyria used a system of eponyms to identify each year for over a thousand years.
• The year was named after the limmu, a high official chosen by lot to serve as the presider over the Akitu festival (celebrating the Mesopotamian new year).
• The earliest recorded limmu eponyms were from the Assyrian trading colony at Karum Kanesh, dating back to the beginning of the 2nd millennium BC.
• The system continued to be used until the end of the Neo-Assyrian period around 612 BC.
• Assyrian scribes compiled limmu lists with an unbroken sequence of almost 250 eponyms from the early 1st millennium BC, which is an invaluable chronological aid.

Olympiad Dating

• The ancient Greeks used the Olympic Games as a way to indicate the passage of years.
• The Olympic Games provided a mutually recognizable system of dates among the independent city-states.
• The system was not used in everyday life and was in use from the 3rd century BC.
• The modern Olympic Games do not continue the four-year periods from ancient Greece.

Indiction Cycles

• The indiction cycle was a common system used to date events and documents.
• 15 indictions made up an agricultural tax cycle in Roman Egypt, with an indiction being a year in duration.
• Documents and events began to be dated by the year of the cycle in the 4th century.
• This system was used long after the tax ceased to be collected and was used in Gaul, Egypt until the Islamic conquest, and in the Eastern Roman Empire until its conquest in 1453.

Seleucid Era

• The Seleucid era was used in much of the Middle East from the 4th century BC to the 6th century AD.
• The era was computed from the epoch 312 BC when Seleucus I Nicator captured Babylon.
• The Seleucid era continued to be used until the 10th century AD among Oriental Christians.

Ancient Rome

• Consular Dating: an early and common practice involved naming both consuls ordinarii who had taken up the office on 1 January of the relevant civil year.
• The use of consular dating ended in AD 541 when the emperor Justinian I discontinued appointing consuls.
• Dating from the Founding of Rome: another method, rarely used, was ab urbe condita or anno urbis conditae (AUC), which counted the years from the founding of Rome.
• The system was introduced by Marcus Terentius Varro in the 1st century BC, with the first day of its year being Founder’s Day (21 April).
• It was rarely used in the Roman calendar and was mostly replaced by naming the two consuls that held office in a particular year.

Middle Ages Period

The section is about the different traditional calendar eras that are in use today and were introduced during the transition from Late Antiquity to the Early Middle Ages, which took place between the 6th and 10th centuries. The following are the different calendar eras discussed in the article:

• Christian era: The Etos Kosmou of the Byzantine Calendar, which places Creation at the beginning of its year 1 (5509 BC), was first used in the 7th century AD. The Era of Martyrs or Era of Diocletian is reckoned from the reign of Roman Emperor Diocletian and the first year of this era was 284/5.
• Incarnation Era: Used by Ethiopia with its epoch being 29 August, AD 8 in the Julian calendar.
• Armenian calendar: The era is fixed at AD 552.
• Dionysian “Common Era”: Introduced by Dionysius Exiguus in 525 and is based on the Incarnation of Christ. The beginning of the numbered year varied from place to place.
• A.D. (or AD): The Latin Anno Domini meaning “in the year of (our) Lord” is the dominant Western Christian Era used in the Gregorian calendar.
• B.C. (or BC): Meaning “Before Christ” and used for years before AD 1.
• C.E. (or CE) and B.C.E. (or BCE): Meaning “Common Era” and “Before the Common Era”, respectively, numerically equivalent to AD and BC.
• Dionysian-derived: Astronomical year numbering equates its year 0 with 1 BC.
• Human era (Holocene era): Proposed by Cesare Emiliani and adds 10,000 to AD years.
• Anno Lucis of Freemasonry: Adds 4000 years to the AD year.
• Islamic: A.H. (or AH) meaning “in the year of the Hijra” is used in the Islamic calendar and denotes the number of years since Muhammad’s emigration from Mecca to Medina in 622.

Modern Period

The French Republican Calendar was in use during the Republican Era of France, which was dated from September 22nd, 1792, the day the French First Republic was proclaimed. It was used from October 24th, 1793, to December 31st, 1805, based on the Gregorian calendar. On the other hand, the Positivist Calendar of 1844 uses 1789 as its starting point.

The Republic of China, now known as Taiwan, has been using the Republican era since 1912, which is the first year of the republic. This is the same as the Juche era used in North Korea, starting from the year of its founder Kim Il-Sung’s birth.

The Italian Fascists instituted the “Fascist Era” or “Era Fascista”, denoting the years since the March on Rome in 1922, using Roman numerals. For example, 1934 was represented as XII E.F. This era was abolished in 1943, with the fall of fascism in Italy, but was restored in the northern part of the country during the Italian Social Republic. The Gregorian calendar was still in use, and the year was shown using both Arabic numerals (for the Common era) and Roman numerals (for the Fascist era). The Fascist calendar year started on October 29th, so for instance, October 27th, 1933, was represented as XI E.F., while October 30th, 1933, was represented as XII E.F.

China historically used the regnal year of its emperors, known as the Chinese era name. While most Chinese do not assign numbers to the years of the Chinese calendar, a few, such as expatriate Chinese, use a continuous count of years from the reign of the legendary Yellow Emperor, starting from 2698 BC as year 1. The Chinese years 4637, 4697, or 4698 started in the early 2000s.

In Korea, from 1952 to 1961, years were numbered using the Dangi years, with 2333 BC considered as the first year. The Assyrian calendar, introduced in the 1950s, has its era fixed at 4750 BC.

The Japanese calendar is based on the current Emperor of Japan’s reign. The current emperor took the throne in May 2019, marking the start of the Reiwa era, previously known as Heisei 31.

The United States government sometimes uses a calendar of its Independence era, which is fixed on July 4th, 1776. This is used together with the Anno Domini civil calendar. For example, the U.S. Constitution is dated “September 17th, 1787, in the Year of our Lord one thousand seven hundred and Eighty seven and of the Independence of the United States of America the Twelfth.” Presidential proclamations are also dated in this manner.

In the field of religion, Scientology uses the “After Dianetics” (A.D.) era, which is numbered relative to the first publication of the book “Dianetics: The Modern Science of Mental Health” in 1950. The Discordian calendar uses the “Year of Our Lady of Discord” (Y.O.L.D.) designation for the year number, starting from January 1st, 1166 BC, in the Discordian year 0.

In terms of practical use, the “Before Present” (B.P.) era is used for counting radiocarbon years before 1950. The Julian day number is used for counting days, not years, with an epoch fixed at noon on January 1st, 4713 BC, in the proleptic Julian calendar. This is equal to November 24th, 4714 BC, in the proleptic Gregorian calendar.