JS Tip 254: From the Writing Workshops: Using A.M. and a.m. and am and AM

We appreciate your suggestions and questions.

Roger Rice of Mountainland Applied Technology College noticed “the way people show times as AM and PM seems to be all over the place: 9:00 AM vs 9:00 a.m. vs 9:00 am. Capitals and periods and nothing. Please help.”

The Chicago Manual of Style1 suggests using lower-case letters with periods:

      The meeting began at 9:00 a.m.

Everything else—A.M., AM, and am—is wrong. Or, at least, non-standard.

But what time is 12:00 p.m.?   

You probably know a.m. is an abbreviation for ante meridiem (not meridian), Latin for “before midday” (meridiem meaning “midday”). Post meridiem (p.m.) means “after midday.”

But wait a minute.

That would mean twelve hours before midday (ante meridiem or a.m.) would be midnight. And twelve hours after midday (post meridiem or p.m.) would be midnight.


Chicago suggests “numbers should never be used to express noon or midnight.” Notice that: never. Chicago suggests writing out noon and midnight:

The meeting began at 9:00 a.m. and ended at noon.

Rodriguez was born at midnight, August 21-22. 

The only exceptions are with a twenty-four-hour system (1200 as noon and 0000 or 2400 as midnight) or “informally, in an expression like twelve o’clock at night.”

What do you think? Does that help?

Hey. We love this stuff.


1Our source is The Chicago Manual of Style, 16th Edition, Sections 9.39 and 9.40. Look it up. It’s interesting.