October 09, 2014
Poseidon of the East (40-41)
The nengou system (年号), called kokureki (国歴) in the novel, resets to year 1 upon the accession of a new emperor. In the past, an emperor could designate a new nengou whenever the fancy struck him, which Shouryuu seems fond of doing.
Taika (大化) and Hakuchi (白雉) are the earliest recorded nengou in Japanese history, marking the reign of Emperor Kotoku (645-654). Daigen (大元) is also the name of the Great Yuan Empire, founded by Kublai Khan after he conquered China in 1271.
Japanese of Shouryuu's time would have been quite familiar with Kublai Khan, thanks to his two failed invasions of Japan in 1274 and 1281, ultimately foiled each time by the "Divine Wind" or Kamikaze.