Nihon Kōki (日本後紀) is an officially commissioned Japanese history text.
Following the earlier national history Shoku Nihongi (797), in 819 Emperor Saga ordered the compilation of the years since then.
Much of the text was lost during the Ōnin and Bunmei Wars in the late 15th century.
It spans four imperial reigns: Kanmu, Heizei, Saga, Junna.
As such, cherry blossom season in Kyoto has been continuously observed for over twelve centuries, which has proven valuable in studying the timing of the seasons, as well as that of climate change.