noun
first watch of the night (approx. 7pm to 9pm)
Archaic timekeeping term from the traditional Chinese-influenced system dividing the night into five watches. Not used in modern everyday Japanese.
昔の時刻では、甲夜は夜の最初の刻限にあたる。
In the old time system, 甲夜 corresponds to the first period of the night.