In ordinary formal prose, write noon. It is concise and avoids uncertainty about whether 12 p.m. means the middle of the day. Follow the required house style or form if it specifies another notation; otherwise, avoid both “12 p.m.” and the potentially redundant “12:00 noon.”
Why “noon” is the clearest default
The issue is not that every style guide bans “12 p.m.” Rather, major guides use different conventions, and some regard the 12-hour label as ambiguous at the exact boundary between morning and afternoon. In running prose, spelling out noon removes that ambiguity without adding digits.
The 18th edition of The Chicago Manual of Style advises against using numerals for noon or midnight outside the 24-hour system, and calls “12:00 p.m.” and “12:00 a.m.” potentially ambiguous. AP-related guidance says to spell out noon and midnight when the time matters; the University of Illinois Chicago editorial guide also makes noon and midnight exceptions to its general preference for numerals in times.
How the common forms compare
| Form | Use in formal writing | Reason |
|---|---|---|
| noon | Best default in running prose | Clear and concise; avoids the 12-hour boundary ambiguity. |
| 12 p.m. | Use only when required by the governing style or format | Some authorities accept it, but Chicago considers it potentially ambiguous. |
| 12:00 noon | Usually avoid | Some guides consider it redundant. Microsoft says not to use “12:00 noon,” and UIC says not to write “12 noon.” |
| 12:00 | Use only where a 24-hour convention is established | Without that convention, the reader may not know whether it means noon or midnight. |
There is no single notation mandated by every institution. The 2016 U.S. Government Publishing Office Style Manual, for example, lists “12 p.m. (12 noon)” and “12 a.m. (12 midnight).” That is a documented alternative, not proof that all government documents use it. When writing for an organization, journal, client, or fixed form, its stated style takes precedence.
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How to write appointments and deadlines
For a routine appointment in prose, write “The meeting starts at noon.” If the date, location, or local time zone could be misunderstood, include the relevant context as well as the time. In a table or software field, use the format the system or document requires and keep it consistent.
For midnight, specify the date
“12:00 a.m.” alone does not make clear which date is meant: the start of a date or the end of the preceding one. Chicago’s guidance recommends naming the date or date span, such as “midnight, April 12–13,” when that distinction matters. Its example distinguishes 2400 hours at the end of a date from 0000 hours at the start of the following date. For a deadline, state the applicable date explicitly rather than relying on “12 a.m.”
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When to use 24-hour time
A 24-hour clock can be a better fit for technical schedules, international communications, and bilingual documents, particularly when the audience or institution expects it. The Government of Canada guidance on time of day points to 24-hour notation for international or bilingual documents, in line with the standards and government guidance it cites. Use the convention required by the audience; do not mix 12-hour and 24-hour formats without a reason.
Quick Recap
A quick decision rule
- Formal running prose: write “noon.”
- A required style guide or form: follow its exact format, even if it differs from the prose default.
- A deadline at midnight: identify the date or date span so the boundary is unmistakable.
- An international or bilingual schedule: use 24-hour time when that is the audience’s or institution’s convention.
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