Chinese chronicles record a memorial sent by…
478 CE
Chinese chronicles record a memorial sent by the "King of Japan" (possibly the legendary Yūryaku, conventionally considered to have reigned from 456–479), who describes himself as "Supreme Director of Military Affairs in Japan and Korea" to the Court of the Northern Wei Dynasty.
The Tuoba emperor responds by confirming the Japanese dynasty in those titles.
This is the earliest verifiable date in Japanese history.