Shinengo (literally means private nengo; nengo is a name of an era) was the name of an era which was not declared by a dynasty that had a stable administrating power in South East Asia, where the name of an era was used as the method of counting years. It is also called Inengo (different nengo), Ginengo (pseudo nengo), and Sennengo (impertinent nengo). Shinengo were mainly used by rebels and opponents against then existing dynasty, and their usage period were relatively short. In Japan, the nengo written in some historical books as declared by the Emperor but not written in official history, or the nengo which was ancient and its use was seen in antique archaeological materials are called Itsunengo (era name which really existed but taken out of official history) - and they are sometimes classified as Shinengo.
From the point of view that Shinengo had little intention to oppose the dynasty of the time, they are sometimes regarded as different from 'Itsunengo' and 'Sennengo or Setsunengo (stealing of nengo)' which had a clear rebellious intention.
More than forty Shinengo have been confirmed up to now. Most of them were used in the medieval ages. Kyotoku, which Shigeuji ASHIKAGA continued to use as an name of era even after kaigen (changing the name of an era) was declared, can be regarded as Shinengo.
In the Sengoku period (period of warring states), there were some Shinengo widely used in Togoku (the eastern part of Japan, particularly Kanto region)
They are more often seen in Itahi (board monument), Kakocho (a family register of deaths), Junreifuda (pilgrimage tags), and Koshaji engi (writings about the history of old temples and shrines) than in official documents. Though not frequently, but they were used even after the Edo period. The most recent one was 'Seiro' (which literally means to punish Russia), which was in 1904 (Meiji 37).
A list of Shinengo in Japan. Nengo in ancient times.
Shinengo after the medieval times.
There is an interpretation that the Kyushu nengo table of Kyushu dynasty theory (which was nengo historically investigated by Shigenobu TSURUMINE as the nengo of Kumaso [a tribe living in the ancient Kyushu district]) is a kind of Shinengo. The authenticity of Kyushu nengo is not accepted in today's academic society.