零时 is the formal/military way to say 'midnight' (00:00 on the 24-hour clock), especially common in news for policy effective dates and timetables. Everyday speech prefers
半夜 (bànyè) for late-night and
十二点 (shí'èr diǎn) for the clock time.
零点 (língdiǎn) is a near-synonym.
