The latest edition of the Chicago Manual of Style recommends am and pm , with or without periods.
They used to be set in small caps, but that's falling out of favour.
You shouldn't use these abbreviations with the words morning, afternoon, evening, night, or o’clock.
There is no need to spell out times in an essay, although you might in creative writing or prose.
The important thing is to pick a convention, and be consistent.
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According to an article on About.com : Dates, phone numbers, and time: Usenumbers for dates: My birthday is March 16.
He was born on Valentines Day, 1975.
And use numbers for phone numbers: The phonenumber for the school is 800-555-6262
And use numbers for time if using a.m. or p.m.:
The alarm will sound at 7 p.m. I make my bed at 7 a.m. eachmorning.
But spell out times whenusing "o'clock" or when the a.m. orp.m.
are omitted: The alarm will soundat seven o'clock.
I make my bed atseven each morning.
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Several conventions exist to abbreviate ante meridiem and post meridiem - with or without periods and with capital or small letters.
11 am, 11 a.m., 11 AM, 11 A.M.
The choice is made based on regional or typographical factors.
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"[...] departs at 10:48 P.M." is written in William Strunk Jr. and E.B. White's The Elements of Style .
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The correct way to write a precise time when using the acronym of Ante or Post Meridiem is AM or PM in professional writing.
Sources: Common Errors in English Usage
The time stamp on your email, mobile phone, digital clock, Microsoft Windows, etc.
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“A quarter of/to/till midnight” or “Eleven forty-five in the evening”
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When I am writing an essay, do I spell out times? How would I write AM or PM?  Example:     11:45 PM   How would I write that?
Do I spell out a time in an essay?
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<orthography><time>
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It is generally agreed that the correct way for writing Ante or Post Meridiem in professional writing is with "am" and "pm", with or without periods. This is recommended by the Chicago Manual of Style. However, there is no clear rule as to whether this should be written in capitals or lower case. The latest edition of the  Chicago Manual of Style  recommends  am  and  pm , with or without periods.They used to be set in small caps, but that's falling out of favour. One shouldn't use these abbreviations with the words morning, afternoon, evening, night, or o’clock. There is no need to spell out times in an essay, although you might in creative writing or prose. Consistency is the key.  Ue numbers for time if using a.m. or p.m. as in " The alarm will sound at 7 p.m.  I make my bed at 7 a.m. each morning".  Some believe that there are several optional conventions and can be ased on regional or typographical factors.
It is generally agreed that the correct way for writing Ante or Post Meridiem in professional writing is with "am" and "pm", with or without periods. This is recommended by the Chicago Manual of Style. However, there is no clear rule as to whether this should be written in capitals or lower case. The preferred abbreviation depends on the style guide.
Several conventions exist to abbreviate  ante meridiem  and  post meridiem  - with or without periods and with capital or small letters.  The latest edition of the  Chicago Manual of Style  recommends  am  and  pm , with or without periods. Others maintain that the correct way to write a precise time is AM or PM.   The choice may be made based on regional or typographical factors. However, some say that here is no need to spell out times in an essay, although you might in creative writing or prose. The best way to write times may depend on regional or typographical factors and there may even be no need to spell them out at all. Similarly, there is no clear agreement regarding how to write ante meridiem or post meridiem, with suggestions including lower case, upper case and with or without periods. You might consider writing "11:45 PM" as "Eleven forty-five in the evening".
The correct way to write a precise time when using the acronym of Ante or Post Meridiem is AM or PM in professional writing.
The correct way to write a precise time when using the acronym of Ante or Post Meridiem is AM or PM in professional writing.
The latest edition of the  Chicago Manual of Style  recommends  am  and  pm , with or without periods. They used to be set in small caps, but that's falling out of favour. You shouldn't use these abbreviations with the words morning, afternoon, evening, night, or o’clock There is no need to spell out times in an essay, although you might in creative writing or prose.   Be consistent  And use numbers for time if using a.m. or p.m.:   There are several conventions. The choice is made based on regional or typographical factors.
S0: The latest edition of the  Chicago Manual of Style  recommends  am  and  pm , with or without periods. S0: Several conventions exist to abbreviate  ante meridiem  and  post meridiem  - with or without periods and with capital or small letters.    S0: The correct way to write a precise time when using the acronym of Ante or Post Meridiem is AM or PM in professional writing.   
The best way to write out times may well depend on regional or typographical factors. There is no need to spell out times in an essay. There are several ways of writing ante meridiem and post meridiem, though the Chicago Manual of Style recommend lower case without periods. The correct way to write ante meridiem or post meridiem in professional writing is AM or PM. You could write 11:45 PM as "Eleven forty-five in the evening".