November 978 AD
February 20, 978 Tengen gannen (天元元年): The new era name was created to mark an event or a number of events. The previous era ended and a new one commenced in Jōgen 3, on the 15th day of the 4th month of 976.
978 (Tengen 1, 8th month): The emperor allowed the daughter of Fujiwara no Kaneie to be introduced into his household; and shortly thereafter, she gave him a son.
978 (Tengen 1, 10th month): Fujiwara no Yoritada was elevated to the position of Daijō-daijin; Minamoto no Masanobu was made Sadaijin; and, Fujiwara no Kaneie was made Udaijin.
Tengen (era) Wikipedia
Tengen (天元) was a Japanese era (年号, nengō, "year name") after Jōgen and before Eikan. This period spanned the years from November 978 through April 983. The reigning emperor was En'yū-tennō (円融天皇).