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.
日本では、正史には記載されていないが、天皇が定めたものとして一部の史書に記載があったり、考古資料に使用例が見られる古代の年号を逸年号と呼び、これに含める場合がある。