Annals of the Joseon Dynasty
Emperor Dangun's rule is usually calculated to begin in 2333 BCE, based on the description of the "Dongguk Tonggam" (1485) contrary to the 40th year of the reign of the legendary Chinese Emperor Yao. Other sources vary somewhat, but also put it during Yao's reign (traditional dates: 2357 BC-2256 BC). The "Samguk Yusa" states Dangun ascended to the throne in the 50th year of Yao's reign, while "Annals of the Joseon Dynasty" says the first year and "Dongguk Tonggam" says the 25th year.