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A regnal year is a year of the reign of a sovereign, from the Latin regnum meaning kingdom, rule. Regnal years 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.
Applying this ancient epoch system to modern calculations of time, which include zero, is what led to the debate over when the third millennium began. Regnal years are "finite era names", contrary to "infinite era names" such as Christian era, Jimmu era, Juche era, and so on.
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. Ancient Egyptian chronology was also dated using regnal using. The Zoroastrian calendar also operated with regnal years following the reform of Ardashir I in 3rd century.
The Canon of Kings is a list that dates the reigns of various Babylonian, Persian, Macedonian, Egyptian, and Roman monarchs, that was used by ancient astronomers as a way to date astronomical phenomena. The Liberian Catalogue is a similar list of popes of Early Christianity, that was used to date early events in the religion's history.
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.[clarification needed]
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 era names (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 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.
The current emperor, Naruhito succeeded to the throne on 1 May 2019, after his father Akihito abdicated the throne, citing age and poor health. The name of his era is Reiwa, which was formally announced by the Government of Japan a month before Naruhito succeeded the throne, on 1 April 2019. Therefore, 1 May 2019 is considered the beginning of the Reiwa 1.
The former emperor, Akihito, succeeded the throne on 7 January 1989 on the death of his father Emperor Shōwa, with the name Heisei decreed as the name of his regnal era by the Cabinet. Thus the year 1989 corresponds to Heisei 1 (平成元年 Heisei gannen, or "first year").
The use of era names was common throughout the various historical states that occupied the Korean peninsula. 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.
During the Joseon Dynasty, Korea used Chinese era names as a demonstration of its respect and loyalty to Ming and Qing dynasty of China. Even after the Ming Dynasty was replaced by Qing, Koreans continued to use the Ming era names, using the era name of the last Ming emperor, the Chongzhen Emperor, after his death in 1644, and continued to do so for nearly 200 years. However, this was done mostly privately, because of the pressure exerted by the Qing government.
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.
Regnal years continues to see limited use in the Commonwealth realms, a group of sixteen sovereign states that share the same monarch. The present conventions for regnal years in the Commonwealth realms originates with the Kingdom of England, which used regnal years to date its public documents. The start of a new regnal era in the English regnal dating system originally began on the date of the monarch's coronation. However, the system was changed in 1307 to begin on the date the monarch succeeds the throne, beginning with the ascension of Edward II.
The regnal years used throughout the Commonwealth realms are identical to one another, as they share the same line of succession. The present monarch, Elizabeth II, became the sovereign on 6 February 1952, after the death of her father George VI. Thus, 6 February 1952 is considered the beginning of the year 1 Eliz. 2; with the last day of each Eliz. 2 regnal year being 5 February.
The regnal dating system is used in the numbering system for all Acts of the Parliament of Canada. All Acts are given an individual chapter number, assigned by its numeric order of when it received royal assent, along with the regnal year, and the name of the reigning Monarch of Canada. The use of regnal years in legal citations is acceptable in Canadian legal practices, although usage of the Gregorian calendar is more common.
The use of the regnal year was used throughout the legislative sessional volumes of the Legislative Assembly of Ontario, until it was replaced by the calender year in 1949. However, the regnal year continues to be used on the title pages of the legislature's sessional volumes.
The use of regnal years in the United Kingdom originated in England, a country of the United Kingdom. The regnal dating system was used to date documents of parliamentary sessions until 1963, when it began to date its documents using the Gregorian calendar. The change to the Gregorian calendar was legislated under the Acts of Parliament Numbering and Citation Act 1962.
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]." 2019 is the 244th 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".
- "Emperor Akihito: Japanese monarch declares historic abdication". BBC News. British Broadcasting Corporation. 30 April 2019. Retrieved 1 May 2019.
- Arnold-Baker, Charles (2015). The Companion to British History. Routledge. ISBN 1-3174-0039-9.
- Bishop, Olga B. (2016). Canadian Official Publications: Guides to Official Publications. Elsevier. p. 117. ISBN 1-4831-5523-4.
- Flaherty, David H. (1981). Essays in the History of Canadian Law, Volume 1. University of Toronto Press. ISBN 1-4875-9697-9.
- 112 H.R. 2