Wareki (Japanese Calendar) (和暦)

Wareki (Japanese calendar) refers to a calendar peculiar to Japan, by which time is divided into periods based on gengo (an era name) and ensuing years
It started with gengo 'Taika' in the reign of Emperor Kotoku in the Asuka period and has been used for fifteen centuries.

Currently, the Era Name Law stipulates that gengo can be changed only when a heir apparent becomes emperor (the One Era for One Emperor system).

Prior to the Meiji period, gengo were changed whenever an ominous event took place, or infectious disease spread.

Because most gengo lasted for a very short time, from one year to a dozen years at most, there exists 250 gengo up to the Heisei era.

In Japan, almost all the documents submitted to public offices such as administrative institutes (public documents) allow people to use only the Japanese calendar when writing a year.

As many gengo exist, critics point out that the Japanese calendar is unpractical for the following reasons:
Compared with the western calendar, which continues in chronological order, the Japanese calendar requires people to remember all the gengo or to check them in a table to have a clear picture of historical events.

When gengo is changed in the case of the accession of the heir apparent to the throne, it is impossible to accurately express the point when exactly one year has passed from the succession.

Gengo may be changed at any point of a particular year (Take 1989 for example, when the Showa era ended on January 7 (January 1 to January 7: 64th year of the Showa era), followed by the Heisei era (January 8 to December 31: the first year of the Heisei era).

As two gengo coexist in the same year of the western calendar, clerical handling becomes cumbersome in some cases.

Every time gengo is changed, enormous clerical work (reprinting of new gengo for public documents, etc.) takes place. On the other hand, gengo in the Japanese calendar can be a convenient tool of representing--though ambiguously--the zeitgeist of a particular epoch and contemporary historical backdrop, such as 'Taisho Retro' and 'the Showa Generation,' and of fortifying literary expressions.

History
The Japanese calendar started with the Taika no Kaishin (the Great Reformation of the Taika Era).

During the period of the Northern and Southern Courts (Japan), two different gengo coexisted simultaneously.

Media companies that use mainly the Japanese calendar and gengo
The Sankei Shimbun Co., Ltd. (Mainly Sankei Shimbun newspapers. Sankei Express, (a type of newspaper) published by the company, tends to use the western calendar).

Tokyo Sports
Japan Broadcasting Corporation (NHK)
Particularly in news programs, NHK tends to use only gengo for domestic news and only the western calendar for overseas news, but in some cases, both gengo and the western calendar are used simultaneously, which often results in confusion.

Calendars adopted in Japan and the starting points of years used in Japan
Japan's old lunisolar calendars
Genka reki (Genka calendar)
Giho reki (Giho calendar)
Taien reki (Taien calendar)
Goki reki (Goki calendar, or Wuji calendar)
Senmyo reki (Senmyo calendar)
Jokyo reki (Jokyo calendar)
Horyaku reki (Horyaku calendar)
Kansei reki (Kansei calendar)
Tenpo reki (Tenpo calendar)
Western calendar
Gregorian calendar

Starting point of years
Anno Domini
Jinmu Tenno Sokui Kigen (Japan's starting point of years upon the accession of Emperor Jinmu to the throne)