A regnal year is a year of the reign of a sovereign, from the Latin 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.
Early use
In ancient times, calendars were counted in terms of the number of years of the reign of the current monarch. Reckoning long periods of time required a king list. The oldest such reckoning is preserved in the Sumerian king list. Ancient Egyptian chronology was also dated using regnal years. 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.
Sinosphere era names
Regnal years were generally used for year marking in the Chinese cultural sphere before the advent of era names. In China, the continuous use of era names began in 140 BC, during the rule of the Emperor Wu of Han. Prior to that, years were usually marked as regnal years of the monarch.
Since 140 BC, era names served as titles for the purpose of numbering and identifying years. Era names were used for over two millennia by Chinese emperors and are still used in Japan.
The Lanfang Republic era, Republic of Formosa era and Republic of China calendar are systems adapted from the traditional era name system, but they are not effectively era names. The and Juche era are based on the year of birth of the thinker or eternal president, respectively. The , and kōki were counted in terms of the number of years of the reign of the first monarch.
As a result of Chinese cultural influence, other polities in the Sinosphere—Korea, Vietnam, Japan, and Ryukyu—also adopted the concept of era name.
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 , 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. (240) was used on Japanese bronze mirrors. (766) and (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.
The short lived , post Zhang Xianzhong, used the Ganzhi calendar without era names. Overseas Chinese used Longfei () or Tianyun ().
Chinese
Chinese era names (, ) were used since 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.
Japanese
The official Japanese system or 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.