Editing Japanese calendar (section)

===Japanese Imperial Years (''kōki'' or ''kigen'')===

The [[Japanese imperial year]] ({{nihongo krt||皇紀|kōki}} or {{nihongo krt||紀元|kigen}}) is based on the date of the legendary founding of Japan by [[Emperor Jimmu]] in 660 BC.<ref>Bramsen, [https://archive.org/details/bub_gb_mk8aAAAAIAAJ/page/n19 p. 11].</ref> For instance, 660 BC is counted as ''Kōki'' 1.

It was first used in the official calendar in 1873.<ref>See "2533 years since Jinmu's accession" in the heading [http://www.ndl.go.jp/koyomi/e/history/pic10_2_lar.html] {{Webarchive|url=https://web.archive.org/web/20130122230000/http://www.ndl.go.jp/koyomi/e/history/pic10_2_lar.html|date=January 22, 2013}}"</ref> ''Kōki'' 2600 (1940) was a special year. The [[1940 Summer Olympics]] and [[world expo|Tokyo Expo]] were planned as anniversary events, but were canceled due to the [[Second Sino-Japanese War]]. The Japanese naval [[Mitsubishi A6M Zero|Zero Fighter]] was named after this year. After the [[World War II|Second World War]], the [[USFJ|United States occupied Japan]], and stopped the use of ''kōki'' by officials.<ref name="auto7">"kigen" in ''Kokushi Daijiten'', vol. 4 (Yoshikawa Kôbunkan, 1983).</ref>

Today, ''kōki'' is rarely used, except in some judicial contexts.<ref name="auto7"/> Usage of ''kōki'' dating can be a [[Japanese nationalism|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 year]]s<ref>{{nihongo2|閏年ニ關スル件}} (Japanese Imperial [[Edict]] No. 90, May 11, 1898)</ref> 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&nbsp;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.