The earliest document was "Saikyuki" (exemplary book on Heian rituals) written in the 10th century, and it says that Emperor Tenchi 'was enthroned in December' in the 10th year of Emperor Tenchi.
もっとも早いのは10世紀に書かれた『西宮記』で、天智天皇10年の「12月に帝位に即く」とある。