零時・０時|れいじreiji[1]atamadaka (head-high)

1. nountwelve o'clock; midnight; noon

• Synonym１２時じゅうにじNeutralTwelve o'clock; covers both noon and midnight, with context deciding which. 「零時」 is the zero-hour count, most often the midnight that starts the day, as in 「午前零時」.
• Related真夜中まよなかNeutralThe dead of night — the deepest, most advanced point of the night, or that stretch of time. Neutral in register.
• Related夜半やはんWrittenMiddle of the night; a written, formal word, heard often in weather forecasts.
• RelatedミッドナイトNeutralLoanword for midnight; mostly used in song titles and product names.
• Related正子しょうしWrittenA written astronomical term for the exact zero hour of midnight; not used in everyday speech.
• Related深更しんこうWrittenDeep night; a written/formal word, as in 「深更に及ぶ」.
• Antonym正午しょうごNeutralNoon, twelve in the daytime — the opposite of the midnight zero hour.

Use 「零時」 for the zero hour that starts the day, as in 「午前零時」. Say 「１２時」 for twelve o'clock, 「真夜中」 for the dead of night, 「夜半」 or 「深更」 in formal prose, 「ミッドナイト」 in names, 「正子」 in astronomy, and 「正午」 for noon.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. 深夜しんやの24時じ。
2. 昼ひるの12時じ。