Ante meridiem (a.m.) is a Latin phrase which means before midday. On the other hand, post meridiem (p.m.) means after midday.
a.m. stands for = Ante meridiem
p.m. stands for = Post meridiem
What AP Style Suggests?
According to this particular style, you cannot capitalize the abbreviations a.m. and p.m. You should abbreviate ante meridiem and post meridiem with lowercase letters.
a.m. , p.m. = Correct
A.M. , P.M. = Wrong
• The presentation will be at 11 a.m.
Important Points to Remember:
1. You should not use unnecessary zeroes. Like,
11:00 p.m. (Wrong)
11 a.m. (Correct)
2. There is no need to write morning or evening with a.m. or p.m. For instance,
• Be ready at 10 a.m. Tuesday morning. (Wrong)
• Be ready at 10 a.m. Tuesday. OR Be ready for Tuesday at 10 a.m. (Correct)