Giho reki (Giho calendar) (儀鳳暦)

Giho reki is a Chinese calendar and is a lunisolar calendar which was compiled by Chunfeng LI, an astronomer during the Tang dynasty. It was originally called Rintoku reki (Linde Calendar) in China. With teisakuho (true conjunction) employed, it is considered a brilliant calendar. It was in this calendar that shinsaku (advance Saku (the solar-lunar conjunction) on the next day) was first adopted.

Rintoku reki of the Tang was used for 73 years from 665 to 728.

In Japanese Calendar
It was introduced to Japan during the Giho years, thus being called Giho reki. But if it had been directly imported from Tang, it would have used the name Rintoku reki, which was then in effect in Tang, so there is a theory that the calendar came from Shidara, whose calendar is thought to have been adopted by King Munmu, just before, in 674.
(In "Nihonkoku genzaisho mokuroku" (Catalogue of Present Books in Japan), made in the Heian period, Rintoku reki and Giho reki were two different calendars, each having its own entry.)

Genka reki (Genka calendar) was adopted in combination with Giho reki in 690, according to "Nihonshoki" (Chronicles of Japan)
Initially, however, it was used primarily to calculate a solar eclipse, and the dates in "Nihonshoki" were also in Ganka reki.
(Since the accession of Emperor Mommu, August 1st was dated differently, with Ganka reki used in "Nihonshoki" and Giho reki in "Shoku Nihongi" (Chronicle of Japan Continued), which makes it appear that there are two theories on the dates, but they were actually identical.)

Five years later, in 697, Giho reki was used alone, although some believe that it was in 696, the previous year, or in 698, the following year. It is believed, however, that shinsaku, one of the characteristics of the new calendar, was not implemented. It was used for another 67 years and the calendar was changed to Taien reki (Taien calendar) in 764.

[Original Japanese]