Midnight
Twelve O'clock
noun
Zero o'clock is 0:00. That's midnight or twelve o'clock.
れいじ
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
零時頃、彼女が町中を一人で猛スピードで歩いているのを見かけました。
I saw her walking really fast downtown by herself around midnight.