Records of heads of state
Officially, the current Emperor of Japan, Akihito is the 125th in line from the first emperor Jimmu, who is variously believed to have reigned in the 1st or 7th century BC. The earliest documentary evidence is only for the 29th emperor, Kinmei (AD 509–571); however, this is sufficient such that even the most conservative of estimates still places the Japanese imperial family as among the oldest lines in the world today.
Regnal year
The official Japanese system or numbers years from the accession of the current emperor, regarding the calendar year during which the accession occurred as the first year. The current emperor Akihito succeeded to the throne on 7 January 1989 on the death of his father Emperor Shōwa, and the new era name Heisei was decreed by the Cabinet. Thus the year 1989 corresponds to . The system was in use sporadically from 645 and continuously from 701. Until 1867 several era names were used during each emperor's reign. From 1868 only one has been used by each emperor. Since 1868 each emperor has been known posthumously by his era name.