
The Japanese naval Zero Fighter was named after this year. The 1940 Summer Olympics and Tokyo Expo were planned as anniversary events, but were canceled due to the Second Sino-Japanese War. Usage of kōki dating can be a nationalist signal, pointing out that the history of Japan's imperial family is longer than that of Christianity, the basis of the Anno Domini (AD) system. Imperial year 1 ( Kōki 1) was the year when the legendary Emperor Jimmu founded Japan – 660 BC according to the Gregorian Calendar. The imperial year system ( kōki) was used from 1872 to the Second World War. The nengō system remains in wide use, especially on official documents and government forms. Beginning with Meiji (1868–1912), each reign has been one era, but many earlier Emperors decreed a new era upon any major event the last pre-Meiji Emperor's reign (1846–1867) was split into seven eras, one of which lasted only one year. Years are numbered within regnal eras, which are named by the reigning Emperor. The official dating system known as nengō ( 年号) (or, strictly speaking, gengō ( 元号)), has been in use since the late 7th century. For the given name, see Kōki (given name). Nowadays, Japanese people know it as well as the regnal eras. The Western Common Era ( Anno Domini) ( 西暦, seireki ) system has gradually come into common use since the Meiji period.However, it never replaced era names, and since World War II has been abandoned. It was first used in the official calendar in 1873. The Japanese imperial year ( 皇紀, kōki, or 紀元 kigen) is based on the date of the legendary founding of Japan by Emperor Jimmu in 660 BC.It is also in general use in private and personal business.

Nengō are the official means of dating years in Japan, and virtually all government business is conducted using that system. Since the Taishō Emperor's ascension in 1912, each emperor's reign has begun a new era before 1868 era names were often also declared for other reasons. The era name ( 元号, gengō) system was also introduced from China, and has been in continuous use since AD 701.Now, though, the cycle is seldom used except around New Year. It was often used together with era names, as in the 1729 Ise calendar shown above, which is for "the 14th year of Kyōhō, tsuchi-no-to no tori", i.e., 己酉. The Chinese sexagenary cycle was introduced early into Japan.Japan has had more than one system for designating years: In Japan today, the old Chinese calendar is virtually ignored celebrations of the Lunar New Year are thus limited to Chinese and other Asian immigrant communities. But in 1873, as part of Japan's Meiji period modernization, a calendar based on the solar Gregorian calendar was introduced.

After that, Japan calculated its calendar using various Chinese calendar procedures, and from 1685, using Japanese variations of the Chinese procedures. The lunisolar Chinese calendar was introduced to Japan via Korea in the middle of the sixth century.
