Japanese era names

The Japanese era name (年号, nengō^?, "year name"), also known as gengō (元号^?), is the first of the two elements that identify years in the Japanese era calendar scheme. The second element, a number, counts the years since the era began; as in many other systems, there is no year zero. For example, the first year of the Heisei period was 1989 AD/CE, or "Heisei 1," so the year 2017 CE in this scheme is "Heisei 29".

As elsewhere in East Asia, the use of nengō was originally derived from Chinese Imperial practice, although the Japanese system is independent of the Chinese, Korean, and Vietnamese era-naming systems. Unlike some of these other similar systems, Japanese era names are still in use. Government offices usually require era names and years for official papers.

The four 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 64 years, Shōwa is the longest era to date.

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

...
Wikipedia