Minamoto no Yoritomo becomes Sei-i Taishōgun and the de facto ruler of Japan. (Traditional Japanese date: the 12th day of the seventh month in the third year of the Kenkyū (建久) era).

Minamoto no Yoritomo was the founder and the first shogun of the Kamakura shogunate, ruling from 1192 until 1199, also the first ruling shogun in the history of Japan. He was married to Hōjō Masako, who acted as regent (shikken) after his death.

Shogun , officially seii taishōgun , was the title of the military rulers of Japan during most of the period spanning from 1185 to 1868. Nominally appointed by the Emperor, shoguns were usually the de facto rulers of the country, except during parts of the Kamakura period and Sengoku period when the shoguns themselves were figureheads, with real power in the hands of the shikken (執権) of the Hōjō clan and kanrei (管領) of the Hosokawa clan. In addition, Taira no Kiyomori and Toyotomi Hideyoshi were leaders of the warrior class who did not hold the position of shogun, the highest office of the warrior class, yet gained the positions of daijō-daijin and kampaku , the highest offices of the aristocratic class. As such, they ran their governments as its de facto rulers.

De facto describes practices that exist in reality, regardless of whether they are officially recognized by laws or other formal norms. It is commonly used to refer to what happens in practice, in contrast with de jure. This distinction is highly significant in fields like law and governance.

Japanese calendar types have included a range of official and unofficial systems. At present, Japan uses the Gregorian calendar together with year designations stating the year of the reign of the current Emperor. The written form starts with the year, then the month and finally the day, coinciding with the ISO 8601 standard.