조선왕조실록
Dangun is said to have founded Gojoseon around 2333 BC, based on the descriptions of the "Samgungnyusa", "Jewang Ungi", "Dongguk Tonggam" and the "Annals of the Joseon Dynasty". The date differs among historical sources, although all of them put it during the mythical Emperor Yao's reign (traditional dates: 2357 BC? – 2256 BC?). "Samgungnyusa" says Dangun ascended to the throne in the 50th year of the legendary Yao's reign, "Annals of the King Sejong" says the first year, and "Dongguk Tonggam" says the 25th year.