third division of the night (approx. 11pm to 1am)
Archaic term from the traditional Chinese timekeeping system, dividing the night into five watches. 丙夜 corresponds to the third watch, roughly 11 p.m. to 1 a.m. Not used in modern everyday Japanese.
In classical literature, 丙夜 appears as the third period of the night.
From the Chinese system of dividing the night into five equal watches (五夜). 丙 (third of the ten celestial stems) indicates the third watch.