The Japanese era name (Japanese: 年号, Hepburn: nengō, "year name") or gengō (元号), is the first of the two elements that identify years in the Japanese era calendar scheme. The second element is a number which indicates the year number within the era (with the first year being "gan (元)"), followed by the literal "nen (年)" meaning "year".

Era names originated in 140 BCE in Imperial China, during the reign of the Emperor Wu of Han.^[1][2] As elsewhere in the Sinosphere, the use of era names was originally derived from Chinese imperial practice,^[2][3][4] although the Japanese system is independent of the Chinese, Korean, and Vietnamese era name systems. Unlike its other Sinosphere counterparts, Japanese era names are still in official use. Government offices usually require era names and years for official papers.

The five era names used since the end of the Edo period in 1868 can be abbreviated by taking the first letter of their romanized names. For example, S55 means Shōwa 55 (i.e. 1980), and H22 stands for Heisei 22 (2010). At 62 years and 2 weeks, Shōwa is the longest era to date.

The Reiwa (令和)^[5] era began on 1 May 2019, the day of accession of Naruhito to the throne as the 126th Emperor of Japan,^[6] following the day of the planned and voluntary abdication^[7] of his father, the 125th Emperor, Akihito. Emperor Akihito had received special permission to abdicate,^[8] rather than serving in his role until his death, as is the rule.^[9] The Reiwa (令和) era followed the 31st and final year of the Heisei era (平成31年), which had started on the day after the death of Emperor Hirohito on 8 January 1989.

A graphical timeline is available at Japanese era names

The system on which the Japanese era names are based originated in China in 140 BC, and was adopted by Japan in 645 AD, during the reign of Emperor Kōtoku.

The first era name to be assigned was "Taika" (大化), celebrating the political and organizational changes which were to flow from the great Taika reform (大化の改新) of 645. Although the regular practice of proclaiming successive era names was interrupted in the late seventh century, it was permanently re-adopted in 701 during the reign of Emperor Monmu (697–707). Since then, era names have been used continuously up through the present day.^[10]

The era name system that was introduced by Emperor Kōtoku was abandoned after his death; no era names were designated between 654 and 686. The system was briefly reinstated by Emperor Tenmu in 686, but was again abandoned upon his death about two months later. In 701, Emperor Monmu once again reinstated the era name system, and it has continued uninterrupted through today.

Although use of the Gregorian calendar for historical dates became increasingly common in Japan, the traditional Japanese system demands that dates be written in reference to era names. The apparent problem introduced by the lack of era names was resolved by identifying the years of an imperial reign as a period.^[12]

Although in modern Japan posthumous imperial names correspond with the eras of their reign, this is a relatively recent concept, introduced in practice during the Meiji period and instituted by law in 1979. Therefore, the posthumous names of the emperors and empresses who reigned prior to 1868 may not be taken as era names by themselves. For example, the year 572—the year in which Emperor Bidatsu assumed the Chrysanthemum Throne – is properly written as "敏達天皇元年" (Bidatsu-Tennō Gannen, "the first year of Emperor Bidatsu"), and not "敏達元年" (Bidatsu Gannen, "the first year of Bidatsu"), although it may be abbreviated as such.^[13] By incorporating both proper era names and posthumous imperial names in this manner, it is possible to extend the nengō system to cover all dates from 660 BCE through today.^[14]

In addition to the official era name system, in which the era names are selected by the imperial court, one also observes—primarily in the ancient documents and epigraphs of shrines and temples—unofficial era names called shinengō (私年号, "personal era name"), also known as ginengō (偽年号) or inengō (異年号). Currently, there are over 40 confirmed shinengō, most of them dating from the middle ages. Shinengō used prior to the reestablishment of the era name system in 701 are usually called itsunengō (逸年号).^{[lower-alpha 1]}

Because official records of shinengō are lacking, the range of dates to which they apply is often unclear. For example, the well-known itsunengō Hakuhō (白鳳) is normally said to refer to 650–654 CE; a poetic synonym for the Hakuchi era. However, alternate interpretations exist. For example, in the Nichūreki, Hakuhō refers to 661–683 CE, and in some medieval temple documents, Hakuhō refers to 672–685 CE. Thus, shinengō may be used as an alternative way of dating periods for which there is no official era name.

Other well-known itsunengō and shinengō include Hōkō (法興) (591–621+ CE), Suzaku (朱雀) (686), Entoku (延徳) (1460), Miroku (弥勒) (1506–1507 or 1507–1509) and Meiroku (命禄) (1540–1543).

The most recent shinengō is Seiro (征露) (1904–1905), named for the Russo-Japanese War.

To convert a Japanese year to a Gregorian calendar year, find the first year of the Japanese era name (also called nengō). When found, add the number of the Japanese year, then subtract 1.

Unofficial non-nengō periods (shinengō) before 701 are called itsunengō (逸年号). Pre-Taika chronology intervals include:

• Reign of Emperor Jimmu, 660–581 BC
• Reign of Emperor Suizei, 581–548 BC
• Reign of Emperor Annei, 548–510 BC
• Reign of Emperor Itoku, 510–475 BC
• Reign of Emperor Kōshō, 475–392 BC
• Reign of Emperor Kōan, 392–290 BC
• Reign of Emperor Kōrei, 290–214 BC
• Reign of Emperor Kōgen, 214–157 BC
• Reign of Emperor Kaika, 157–97 BC
• Reign of Emperor Sujin, 97–29 BC
• Reign of Emperor Suinin, 29 BC–AD 71
• Reign of Emperor Keikō, AD 71–131
• Reign of Emperor Seimu, 131–192
• Reign of Emperor Chūai, 192–201
• Regency of Empress Jingū, 201–270
• Reign of Emperor Ōjin, 270–313
• Reign of Emperor Nintoku, 313–400
• Reign of Emperor Richū, 400–406
• Reign of Emperor Hanzei, 406–412
• Reign of Emperor Ingyō, 412–454
• Reign of Emperor Ankō, 454–457
• Reign of Emperor Yūryaku, 457–480
• Reign of Emperor Seinei, 480–485
• Reign of Emperor Kenzō, 485–488
• Reign of Emperor Ninken, 488–499
• Reign of Emperor Buretsu, 499–507
• Reign of Emperor Keitai, 507–534
• Reign of Emperor Ankan, 534–536
• Reign of Emperor Senka, 536–540
• Reign of Emperor Kinmei, 540–572
• Reign of Emperor Bidatsu, 572–586
• Reign of Emperor Yōmei, 586–588
• Reign of Emperor Sushun, 588–593
• Reign of Emperor Suiko, 593–629^{[lower-alpha 2]}
• Reign of Emperor Jomei, 629–645

Post-Taika chronology intervals not covered by the nengō system include:

• Reign of Empress Saimei, 655–662^[105] ... Saimei (period)^[12]
• Reign of Emperor Tenji, 662–672^[106] ... Tenji (period)^[12]
• Reign of Emperor Kōbun, 672–673^[107] ... Kōbun (period) or Sujaku^[12] (or Suzaku)
• Reign of Emperor Tenmu, 673–686^[108] ... Tenmu (period) or Hakuhō period^[12][109]
• Reign of Empress Jitō, 687–697^[110] ... Jitō (period)^[12]
• Reign of Emperor Monmu, 697–701^[111] ... Monmu (period)^[12]

• Calendar
• Japanese calendar
• Japanese imperial year
• Jikkan Jūnishi (sexagenary cycle)
• Regnal name
• Regnal years worldwide