This reveals that the book was written at least later than May 18, 806, when the era changed from the 25th year of the Enryaku era to the first year of Daido, suggesting that the book being written in the first year of Daido is wrong.
このことから、平城天皇即位による改元の806年(延暦25年・大同元年）5月18日以降であることが分かり、「大同元年」説は誤りということが分かる。