The era after the enthronement of Emperor Jimmu (神武天皇即位紀元, Jinmu-tennō sokui kigen), colloquially known as the Japanese imperial year (皇紀, kōki) or "national calendar year" is a unique calendar system in Japan.
[1] It is based on the legendary foundation of Japan by Emperor Jimmu in 660 BC.
Kōki dating was used as early as 1872, shortly after Japan adopted the Gregorian calendar[4] and was popular during the life of the Meiji Constitution (1890–1947).
The Summer Olympics and Tokyo Expo were planned as anniversary events in 1940 (Kōki 2600);[5] but the international games were not held because of the Second Sino-Japanese War.
In Japan today, the system of counting years from the reign of Emperor Jimmu is used in some judicial contexts.