I used to think PM/AM was correct, but at some point, I switched to using p.m./a.m. for reasons I can't recall. I know that in practical, casual writing, people tend to use whatever form is most convenient to them, but I'm curious about what the official usage—should it exist—actually is.
What is the correct or official form for these initialisms?
- Are they capitalized?
- 4 pm
- Is there a space before them?
- 4 PM
- If the correct usage is a.m./p.m., how does this affect punctuation at the end of a sentence?
- Should we meet at 4 p.m.?
- Let's meet at 4 p.m..
(Let me know if these should be split up into individual questions, but I figured that because they were all related to these initialisms in particular, it might be okay to ask them all in one question.)