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 Saimei 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 Saimei period.
Neither Empress Saimei's reign nor the Saimei periodization are included in the list of 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 Empress Saimei's reign (斉明天皇元年 or 斉明天皇1年) is also construed as the first year of the Saimei period (斉明1年).
Read more about this topic: Saimei (period)