Japanese calendar

The Gregorian calendar has been used in Japan since 1 January 1873, but the names of the months are numbers, and the years are numbered in terms of eras which begin with the accession of each emperor. Within a year or so of a new emperor's assuming the throne, the imperial court announces an official auspicious name for the reign. These era names are referred to as nengō.

This practice, which was widespread in Asia, is based on a Chinese model dating to the 2nd century bce. The Japanese adopted it in 645 ce when, after a coup d'etat, the emperor Kōtoku adopted the name Taika, “Great Reform,” as a nengō. Except for the years 673–686 (now referred to as Hakuhō, a shi nengō, or unofficial nengō), nengō have been officially used in Japan ever since. In earlier periods, emperors would sometimes adopt a new nengō in the middle of a reign, to commemorate some great event. In 1872, however, the government adopted the practise of using only one nengō for an emperor's entire reign, and, upon an emperor's death, the nengō becomes the official posthumous name of that emperor. As recently as 1979 (Shōwa 54), the Diet voted to require use of the nengō in official documents.

The first year of an emperor's reign begins on his accession to the throne and ends on December 31; thereafter a new year of the reign begins each January 1. Strictly speaking, days in the first part of the year in which a new emperor takes the throne would be dated with his predecessor's nengō, and of course that is how contemporary records are dated. Scholars, however, have tended to label every day in such a year as year 1 of whatever nengō applied at the year's end. Years prior to the adoption of the nengō system are often dated by the official posthumous name of the emperor and the year of reign. To distinguish such dates from true nengō, the word tennō (“sovereign”) is usually added to them. Unlike nengō dates, the first year of reign is the first full year of reign.

Recent nengō are:

To convert a Japanese year to a Western year, find the year in the common era of the first year of the nengō from a table like the one above, subtract 1, and add the number of the Japanese year. For example Showa 23 would be (1926 − 1 = 1925, 1925 + 23 = 1948).

The Japanese calendars prior to 1873 were derived from the Chinese, and are discussed with them. Japan adopted the Chinese calendar in 604 ce.

Two books give the Western date corresponding to the first day of every Japanese civil month:

Naimushō Chirikyoku.
Sansei sōran.

Yachita Tsuchihashi.
Japanese Chronological Tables from 601 to 1871 ad.
Tokyo: Sophia University Press (Jōchi Daigaku), 1952.

Gaimusho Bunshoka.
Nihon gaikō bunsho: Kindai in'yōreki taishō hyō.
Tokyo: Gaimusho Bunshoka Gaiko Bunsho Hensanshitsa, (1951?)

Gives the corresponding Western date for every day in the Chinese and Japanese lunar calendars between 1700 ce and 1911.

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