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 Mommu 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 time was temporarily abandoned or allowed to lapse. This interval continued during the Mommu period.
Neither Empress Mommu's reign nor the Mommu periodization are included in the list of nengō for this explicit duration of time, which comes after Suchō and before Taihō.
In the post-Taika or pre-Taihō chronology, the first year of Emperor Mommu's reign (文武天皇元年 or 文武統皇1年) is also construed as the first year of the Mommu period (文武1年).
Read more about this topic: Mommu (period)