section_path: History/Japanese Imperial Years (kōki or kigen)
section_name: Japanese Imperial Years (kōki or kigen)
paragraph_text: Japanese Imperial Years (kōki or kigen)
  
  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. For instance, 660 BC is counted as Kōki 1.
  
  It was first used in the official calendar in 1873. Kōki 2600 (1940) was a special year. The 1940 Summer Olympics and Tokyo Expo were planned as anniversary events, but were canceled due to the Second Sino-Japanese War. The Japanese naval Zero Fighter was named after this year. After the Second World War, the United States occupied Japan, and stopped the use of kōki by officials.
  
  Today, kōki is rarely used, except in some judicial contexts. 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.
  
  The 1898 law determining the placement of leap years is officially based on the kōki years, using a formula that is effectively equivalent to that of the Gregorian calendar: if the kōki year number is evenly divisible by four, it is a leap year, unless the number minus 660 is evenly divisible by 100 and not by 400. Thus, for example, the year Kōki 2560 (AD 1900) is divisible by 4; but 2560 − 660 = 1900, which is evenly divisible by 100 and not by 400, so kōki 2560 (1900) was not a leap year, just as in most of the rest of the world.