After 701, the nengō system was always used, even including the present day. Before 701, nengō were not used during the gap years between Hakuchi and Shuch. Also, nengō were not used in another gap between Shuchō and Taihō.
↑Shinengō used prior to the reestablishment of the nengō system in 701 are usually called itsunengō (逸年号). A list of shinengō and more information can be seen in the Japanese Wikipedia page ja:私年号.
↑NengoCalc (645) 大化 Taika, online conversion of Japanese dates into their Western equivalents; calculation is based on tables from Paul Yachita Tsuchihashi. (1952).
Japanese Chronological Tables from 601 to 1872 A. D. (邦暦西暦対照表) and
Reinhard Zöllner (2003), Japanische Zeitrechnung; retrieved 2012-11-14.
↑Murray, p. 402; Hakuhō, also known as Itsunengō; compare Louis-Frédéric, "Hakuhō" at p. 280; Hakuhou jidai, JAANUS (Japanese Architecture and Art Net Users System), 2001; retrieved 2011-11-14.
↑ 17.017.1Brown, p. 270; excerpt, "The eras that fell in this reign were: (1) the remaining seven years of Shuchō [(686+7=692?)]; and (2) Taika, which was four years long [695-698]. (The first year of this era was kinoto-hitsuji [695].) ...In the third year of the Taka era [697], Empress Jitō yielded the throne to the Crown Prince."
C. Perhaps something like the "timeline" graphics created to explain the Nanboku-chō period will be useful for explaining how these years are identified?