The "Nihonshoki" tells us that the story of Taro happened around the late 5th century, during the reign of Emperor Yuryaku, and 300 years later (in this case really 300 years), in the 8th century, he returned to his birthplace.
『日本書紀』は太郎を5世紀後半の雄略天皇のころの話であるといい、300年後（この場合は300年）の8世紀に生まれ故郷に戻るとある。