Heiji
Heiji (平治) was a Japanese/Chinese era name (年号,, nengō, lit. "year name") after Hōgen and before Eiryaku. This period lasted from April 1159 until January 1160. The reigning emperor was Emperor Nijō-tennō (二条天皇).
Heiji (平治) was a Japanese/Chinese era name (年号,, nengō, lit. "year name") after Hōgen and before Eiryaku.  This period lasted from April 1159 until January 1160. The reigning emperor was Emperor Nijō-tennō (二条天皇).

Change of era
January 21, 1159 Heiji gannen (平治元年): The new era name was created to mark an event or series of events. The previous era ended and a new one commenced in Hōgen 4, on the 20th day of the 4th month of 1159.

Events of the Heiji era
January 23, 1159 (Heiji 1, the 3rd day of the 1st month ): The emperor visited his father.
January 19–May 5, 1159  (Heiji 1, 9th-26th day of the 12th month): The "Heiji Rebellion", also known as the "Heiji Insurrection" or the "Heiji War."

Notes
References
Brown, Delmer M. and Ichirō Ishida, eds. (1979). Gukanshō: The Future and the Past. Berkeley: University of California Press. ISBN 978-0-520-03460-0; OCLC 251325323
Nussbaum, Louis-Frédéric and Käthe Roth. (2005). Japan encyclopedia. Cambridge: Harvard University Press. ISBN 978-0-674-01753-5; OCLC 58053128
Titsingh, Isaac. (1834). Nihon Odai Ichiran; ou, Annales des empereurs du Japon.  Paris: Royal Asiatic Society, Oriental Translation Fund of Great Britain and Ireland. OCLC 5850691
Varley, H. Paul. (1980). A Chronicle of Gods and Sovereigns: Jinnō Shōtōki of Kitabatake Chikafusa. New York: Columbia University Press. ISBN 9780231049405; OCLC 6042764

External links
National Diet Library, "The Japanese Calendar" -- historical overview plus illustrative images from library's collection