Emperor Seiwa
The calendar was imported into Japan in 859. The earliest record of this calendar being used in Japan is in the 8th month of the 3rd year of "Jōgan" (861) during the reign of the Emperor Seiwa. It would continue to be used until 1685, after which it would be superseded by the Jōkyō calendar, the first calendar developed specifically for Japan; by that point the calendar was in error by about two days.