Simplify the following text for a Grade 10 native English-speaking person.
Original: The reign of Emperor Kimmei ( 509-571 ) , the 29th emperor of Japan according to the traditional order of succession , is the first for which contemporary historiography are able to assign verifiable dates ; however , the conventionally accepted names and dates of the early emperors were not to be confirmed as `` traditional '' until the reign of Emperor Kammu ( 737 â `` 806 ) , the 50th sovereign of the Yamato dynasty .
Simplified: No certain dates can be assigned to the emperor 's life or reign . The conventionally accepted names and sequence of the early emperors were not to be confirmed as `` traditional '' until the reign of Emperor Kammu , who was the 50th monarch of the Yamato dynasty .