This list of emperors of Japan presents the traditional order of succession. Records of the reigns are compiled according to the traditional Japanese calendar. In the nengō system which has been in use since the late-seventh century, years are numbered using the Japanese era name and the number of years which have taken place since that nengō era started. The sequence, order and dates of the early emperors are almost entirely based on the 8th-century Nihon Shoki, which tried to retroactively legitimize the Yamato dynasty by dating its foundation further back to the year 660 BC. There are several theories as to who the first Japanese ruler to be backed up by historical evidence; Emperor Yūryaku (r. 456–479), Emperor Kinmei (r. 539–571), etc. The term Tennō ('Emperor'), as well as Nihon ('Japan'), wasn't adopted until the late 7th century.