Tenmu period

The Temmu period is a chronological timeframe during the Asuka period of Japanese history. The Temmu period describes a span of years which were considered to have begun in the 1333rd year of the Yamato dynasty.
This periodization is congruent with the reign of Emperor Tenmu, which is traditionally considered to have been from 673 through 686.
Periodization.
The adoption of the Sexagenary cycle calendar ("Jikkan Jūnishi") in Japan is attributed to Empress Suiko in 604; and this Chinese calendar continued in use throughout the Tenmu period.
In 645, the system of was introduced. However, after the reign of Emperor Kōtoku, this method of segmenting time was temporarily abandoned or allowed to lapse. This interval continues during the Tenmu period.
Neither Emperor Tenmu's reign nor the Tenmu periodization are included in the list of "nengō" for this explicit duration of time. The was an unofficial "nengō" during the reign of Emperor Temmu after "Hakuchi" and before "Suchō." The duration of this discrete non-"nengō" timespan lasted for 15 years.
In the post-Taika or pre-Taihō chronology, the first year of Emperor Tenmu's reign (年号天皇元年 or 年号天皇1年) is also construed as the first year of the Temmu period (年号1年).
Non-nengō period.
Non-nengō periods in the pre-Taihō calendar were published in 1880 by William Bramsen. These were refined in 1952 by Paul Tsuchihashi in "Japanese Chronological Tables from 601 to 1872."
The pre-Tahiō calendar included two non-nengō gaps or intervals in the chronological series:
Nengō were not promulgated (or were allowed to lapse) during the gap years between Hakuchi and Shuchō, and in another gap between Shuchō and Taihō.