Chicago style prohibits the use of numerals (12:00 a.m. and 12:00 p.m.) when expressing noon or midnight. Instead solely using the words “noon” and “midnight” are preferred. For example, consider the following sentences,
- Wrong: I usually take lunch at 12:00 p.m. noon.
- Correct: I usually take lunch at noon.
- Wrong: Staying up until 12:00 a.m. midnight is difficult.
- Correct: Staying up until midnight is difficult.
Chicago Style finds the form 12:00 m. (m. meaning meridies or “noon”) acceptable to express noon but says that very few use that form.
Unlike AP Style, when referring to midnight, Chicago prefers writers to avoid any ambiguity by using double dates. For example,
- The invasion took place at midnight, August 21-22.
For a more full discussion on the differences between noon and midnight, check out our full page on the subject.