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AP Style requires a.m. or p.m. -- lower case, separated by periods. There is no need for an additional period if the sentence ends with the time. The briefing began at 2:30 p.m. This would b...
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#2: Initial revision
AP Style requires a.m. or p.m. -- lower case, separated by periods. There is no need for an additional period if the sentence ends with the time. > The briefing began at 2:30 p.m. This would be the correct format for anything journalistic (newspaper, magazine, wire service, etc.) In addition, many other publications and websites will ask for things to be written in AP Style. The Chicago Manual of Style agrees, except it allows for a.m. or p.m. to be written in "small caps" font. In that case, the periods are not necessary. In formal non-fiction writing, using normal capital letters is never acceptable, nor is using lower case letters without periods. For unsolicited novel manuscripts, I've never seen submission guidelines that address the issue. You can check with individual publishers, but I don't think that it matters until you are actually working with them, at which time, they'll give you very detailed specifications for everything you send them in the future.