Periodization
The adoption of the Sexegenary cycle calendar (Jikkan Jūnishi) in Japan is attributed to Empress Suiko in 604; and this Chinese calendar continued in use throughout the Tenji period.
In 645, the system of Japanese era names (年号,, nengō,?, lit. "year name") was introduced. However, after the reign of Emperor Kotoku, this method of segmenting was temporarily abandoned or allowed to lapse. This interval continued during the Tenji period.
Neither the years of Emperor Tenji's reign nor the Tenji period are included in the list nengō for this explicit duration of time, which comes after Hakuchi and before Suchō.
In the post-Taika or pre-Taihō chronology, the first year of Emperor Tenji's reign (天智天皇元年 or 天智天皇1年) is also construed as the first year of the Tenji period (天智1年).
Read more about this topic: Tenji (period)