Japan has been ruled by emperors since antiquity. The sequence, order and dates of the early emperors are almost entirely based on the 8th-century Nihon Shoki, which was meant to retroactively legitimise the Imperial House by dating its foundation further back to the year 660 BC. Emperor Kinmei (r. 539–571) is often considered the first historical emperor, but the first Japanese ruler supported by historical evidence is actually Emperor Yūryaku (r. 456–479), who is mentioned in the 5th-century Inariyama and Eta Funayama Swords. According to Chinese sources, the unification of Japan took place between the 2nd and 3rd centuries.
The terms Tennō ('Emperor', 天皇), as well as Nihon ('Japan', 日本), were not adopted until the late 7th century AD. In the nengō system which has been in use since the late 7th century, years are numbered using the Japanese era name and the number of years which have elapsed since the start of that nengō era.
