Mitsubishi A6M Zero and the Japanese calendar mysteries

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Why's a Zero?

During the 1930s and 1940s, an educated Japanese would reckon dates by three different calendars. For discourse with foreigners, he would follow the western calendar, but for everyday use he'd prefer the Showa calendar, based on the year Hirohito became emperor. ("Showa" means Enlightened Peace, the name -- inappropriate, as matters turn out -- that Hirohito took for himself and his reign when he succeeded his father in 1926.)

For military purposes, however, our educated Japanese would follow the koki calendar, based on the mythical founding of the Japanese dynasty in 660 BC. Here's how the war years are shown in the three styles, with occasional landmark events:

1931 - Showa 6 - Koki 2591 (Japanese army seizes Manchuria)
1932 - Showa 7 - Koki 2592 (Japanese navy raids Shanghai)
1933 - Showa 8 - Koki 2593
1934 - Showa 9 - Koki 2594
1935 - Showa 10 - Koki 2595
1936 - Showa 11 - Koki 2596
1937 - Showa 12 - Koki 2597 (invasion of China)
1938 - Showa 13 - Koki 2598 (Rape of Nanjing)
1939 - Showa 14 - Koki 2599 (border war with Russia)
1940 - Showa 15 - Koki 2600 (occupation of northern Vietnam)
1941 - Showa 16 - Koki 2601 (to war with U.S., Britain, Dutch) 
1942 - Showa 17 - Koki 2602 (Battle of Midway) 
1943 - Showa 18 - Koki 2603 
1944 - Showa 19 - Koki 2604 (B-29s begin to destroy Japan)
1945 - Showa 20 - Koki 2605 (Japan surrenders)

Why is this of interest? Because, starting about 1936, the Japanese began identifying their military equipment by the last digits of its year of adoption, using the koki calendar. The numerical designation was followed by a description of its function. Thus, when the Pacific War began, these were the army and navy fighter planes in service:

Type 96 Carrier Fighter - Mitsubishi A5M, adopted 1936 (2596), with a few examples still in secondary combat units in December 1941 ("Claude" in the Allied code-name system)

Type 97 Army Fighter - Nakajima Ki-27, adopted 1937 (2597), obsolete by December 1941 but still the army's basic fighter ("Nate")

Type 0 Carrier Fighter - Mitsubishi A6M, adopted 1940 (2600) and a tremendous success in China that year; about 400 were available in first-line squadrons when Japan went to war ("Zeke" in the Allied nomenclature, but almost always called the "Zero")

Type 1 Army Fighter - Nakajima Ki-43, adopted 1941 (2601) but still experimental when the war began, with about 40 in two front-line groups (Hayabusa to its pilots, "Oscar" to Americans)

So "Zero" is merely the English translation of the Japanese character for a nul quantity, which was applied to the aircraft because it went into service in 2600. The Japanese called it Rei-sen, short for Rei (Zero) shiki (Type) sentoki (Fighter). Foreigners who like to parade their knowledge sometimes make a half-translation and call the plane "Zero-sen," but this is to conflate two languages. The correct usage is A6M, or Rei-sen—or Zero!

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