Theera after the enthronement of Emperor Jimmu (神武天皇即位紀元,Jinmu-tennō sokui kigen), colloquially known as theJapanese imperial year (皇紀,kōki) or "national calendar year" is a uniquecalendar system inJapan.^[1] It is based on thelegendary foundation of Japan byEmperor Jimmu in 660 BC.^[2]Kōkiemphasizes the long history of Japan and the Imperial dynasty.^[3] The Gregorian year 2026 is Kōki 2686.

Kōki dating was used as early as 1872, shortly after Japan adopted theGregorian calendar^[4] and was popular during the life of theMeiji Constitution (1890–1947). Its use was promoted by the scholars ofkokugaku in the late 19th century.

TheSummer Olympics and Tokyo Expo were planned asanniversary events in 1940 (Kōki 2600);^[5] but the international games were not held because of theSecond Sino-Japanese War.^[6]

TheImperial Japanese Army (IJA, from 1927) andImperial Japanese Navy (IJN, from 1929) used theKōki system foridentification.^[7] For example many Japanese names circaWorld War II use imperial years:

• The IJA'sType 92 battalion gun was called "ninety-two" because its design was completed in 1932, and the 2592nd year since the first Emperor of Japan was 1932 (Kōki 2592).
• Japan's wartime cipher machine was named theSystem 97 Printing Machine for European Characters because it entered service in 1937 (Kōki 2597).
• The Mitsubishi A6M (Navy Type 0 Carrier Fighter), colloquially called the "Zero" by allied forces, entered service in 1940 (Kōki 2600).

TheProclamation of Indonesian Independence (1945) used the imperial year (Kōki 2605).^[8]

In Japan today, the system of counting years from thereign ofEmperor Jimmu is used in some judicial contexts.^[9] The existing law determining the placement ofleap years is based on theKōki years, using a formula that is effectively equivalent to that of the Gregorian calendar.^[10]Kōki is also used inShinto context.

• Epoch
• Japanese calendar
• Japanese era name

• National Diet Library,"The Japanese Calendar"

• Cultural history of Japan
• Time in Japan

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