The imperial year (皇紀 kōki) is an epoch used before WW2. Kōki 1 is the year when legendary Emperor Jimmu founded Japan, 660 BC according to the Gregorian Calendar. This epoch system was adopted in 1872. In terms of nationalism, Kōki emphasizes the long history of Japan and the imperial family because it is a larger number than the Anno Domini year (AD).
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 aircraft 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.