The oldest dating systems were in regnal years, and considered the date as an ordinal, not a cardinal number. For example, a monarch could have a first year of rule, a second year of rule, a third year of rule, and so on, but not a zeroth year of rule.
Reckoning in various cultures
In ancient times, calendars were counted in terms of the number of years of the reign of the current monarch. Reckoning long periods of times required a king list. The oldest such reckoning is preserved in the Sumerian king list.
In England, and later the United Kingdom, until 1963, each Act of Parliament was defined by its serial number within the session of parliament in which it was enacted, which in turn was denoted by the regnal year or years of the monarch in which it fell.
In Canada, acts of Parliament are dated by the session, the Parliament, the regnal year, and the calendar year. So, for example, a bill passed in the second session during the period spanning 2007–2008 would be dated thus: Second Session, Thirty-ninth Parliament, 56–57 Elizabeth II, 2007–2008
While not strictly a regnal year, time in the United States of America can be derived from the Declaration of Independence (July 4, 1776). For example, the U.S. Constitution is dated as signed 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," and Presidential proclamations will often be ended "IN WITNESS WHEREOF, I have hereunto set my hand this [ordinal] day of [month], in the year of our Lord [year], and of the Independence of the United States of America the [year]." 2018 is the 243rd year of the Independence of the United States of America on and after July 4 of that year. Time is also sometimes reckoned in terms (and sessions, if necessary) of Congress; e.g. House of Representatives Bill 2 of the 112th Congress is dated "112th CONGRESS, 1st Session".
Asian era names
Regnal years were generally used for year marking in East Asia before the advent of era names. In China, the first continuous use of era names was in 140BC. Prior to that, years were usually marked as regnal years of the king or emperor.
From 140BC, an era name was assigned as the name of each year by the leader (emperor or king) of the East Asian countries during some portion of their history. The people of the country referred to that year by that name. Era names were used for over two millennia by Chinese emperors and are still used in North Korea, Japan and Taiwan. It could last from one year to the length of the leader's reign. If it lasted more than one year, numbers were appended to the era name. If it lasted the entire length of the leader's reign, then that leader is often referred to by that name posthumously. However, the leader was often given a more complex formal posthumous name as well. It should not be confused with a temple name, by which many leaders are known.
The Lanfang Republic era, Republic of Formosa era and Republic of China era are era names without an emperor. The Confucius era and Juche era are based on the year of birth of the thinker or eternal president. The Huangdi era, Dangun era and kōki were counted in terms of the number of years of the reign of the first monarch. The Tibetan Empire, Kingdom of Khotan, Liao Dynasty, Western Xia, Jin dynasty (1115–1234), Kara-Khitan Khanate, Mongol Empire, Northern Yuan Dynasty, Qing Dynasty, Nguyễn Dynasty, Joseon Dynasty, Bogd Khaanate of Mongolia and North Korea also use non-Chinese era names. Some are transliterations of their Chinese era names. Chinese era names were also employed in other East Asian countries.
Abolished era names may be reused, for example as a means of claiming or denying political legitimacy. An example of this is, that when the Yongle emperor usurped the throne from his nephew he dated the year of his accession as "洪武三十五年", the 35th year of his father, the Hongwu Emperor's reign, i.e. 1402. Hongwu had in fact died in 1398, and the short reign of the Jianwen Emperor, who ruled between 1398 and 1402 was written out of the official record. However, they would sometimes still be used. 景初四年 (240) was used on Japanese bronze mirrors. 廣德四年 (766) and 建中八年 (787) were used in a Western Regions tomb and a document. Kuchlug did not change the era name.
After the Ming Dynasty fell, the Joseon Dynasty still used Chongzhen, and the Kingdom of Tungning still used Yongli regnal years, thus denying the legitimacy of the Qing dynasty, and showing continued allegiance to the Ming regime.
Chinese era names
The Chinese eras (年號, niánhào) were used sporadically from 156 BC and continuously from 140 BC. Until 1367 AD several were used during each emperor's reign. From 1368 AD until 1912 AD only one era name was used by each emperor, who was posthumously known by his era name, which meant that the era name became equivalent to a regnal year. The tradition of Chinese era names survives in the Republic of China's Minguo calendar, with Minguo, the Chinese for Republic, taking the place of the era name.
The official Japanese system or Gengō (元号) numbers years from the accession of the current emperor, regarding the calendar year during which the accession occurred as the first year. The current emperor Akihito succeeded to the throne on 7 January 1989 on the death of his father Emperor Shōwa, and the new era name Heisei was decreed by the Cabinet. Thus the year 1989 corresponds to Heisei 1 (平成元年 Heisei gannen, or "first year"). The system was in use sporadically from 645 and continuously from 701. Until 1867 several era names were used during each emperor's reign. From 1868 only one has been used by each emperor. Since 1868 each emperor has been known posthumously by his era name.
Korea used independent era names whilst reigning in all of its variety of nations. Korean endemic eras were used from 391 to 1274 and from 1894 to 1910. During the later years of the Joseon Dynasty, years were also numbered from the founding of that dynasty in 1393. From 1952 until 1961, years were numbered in Dangi in South Korea, counting from the legendary founding of Gojoseon in 2333 BC.
The tradition of Korean era names survives in the North Korean Juche calendar, with Juche year 1 being 1912 the year of the birth of Kim Il-sung.