What is the grammatically correct way to write p.m. and a.m.?

11:00 a.m. Pacific time is the correct way to write it

I don’t know this for certain, but I strongly suspect that the a.m. v. AM v. am is a style question, and not a grammar question. In other words, completely up to your discretion.

Just make certain that you NEVER write NOON as 12 PM, as I once learned, the hard way.

I had folks showing up at my place, believing that my party would start AT MIDNIGHT.

Always use the word NOON, to avoid any “confusion.”

@JackAdams – any reasonable person should know that 12 p.m. is noon and not midnight.

or maybe that’s just my stubborn unwillingness to cater to the lowest common denominators of society.

Even if they don’t know which is which.
I’m guessing they can figure it’s not going to start at midnight.
Unless yours is the last party of the night? (In a string of parties.)

I think both the a.m. and AM convention are acceptable.
But are probably, as Lapilofu already pointed out, more of a stylistic issue.

I’ve never understood why the clock restarts with 12.
Shouldn’t it restart with 1 at meridiem?

That way you have an entire unbroken block (1–12) that is ante meridiem
and an entire unbroken block (1–12) that is post meridiem.

Nice, no?

Chicago Manual of Style chooses a.m. and p.m.

Modern Language Ass’n says AM and PM.

Not a grammar issue. You select.

here’s a sidebar: no need to include the minutes as in “10:00 a.m.” just simply write it 10 a.m. but if it’s 10:30 a.m., by all means, “10:30 a.m.” i see you “gailcalled!” i was going to write the same thing! but you beat me to it…

you know what? just use military time. people think it’s confusing, but it really is simpler. just do the math. right now, i’m writing this response at 1623 hours. see? no confusion that i’m writing you in the afternoon and not the morning.

Check out this website.
That place is my one true love.

“AM” stands for the Latin phrase Ante Meridiem —which means “before noon”—and “PM” stands for Post Meridiem : “after noon.” Although digital clocks routinely label noon “12:00 PM” you should avoid this expression not only because it is incorrect, but because many people will imagine you are talking about midnight instead. The same goes for “12:00 AM.” Just say or write “noon” or “midnight” when you mean those precise times.

It is now rare to see periods placed after these abbreviations: “A.M.” , but in formal writing it is still preferable to capitalize them, though the lower-case “am” and “pm” are now so popular they are not likely to get you into trouble.

Occasionally computer programs encourage you to write “AM” and “PM” without a space before them, but others will misread your data if you omit the space. The nonstandard habit of omitting the space is spreading rapidly, and should be avoided in formal writing.

I was taught p.m. or PM, i guess for me pm is just too informal. I was having this discussion with my boss today (b.t.w he misspells words all the time on store signs). My boss wrote, “We will be closing at 5 pm. on Saturday For the 4th of July.” I then said, “it’s p.m.” He then replied with, “No it’s not, what school did you go to?” Well, being that i went to a University and got a degree in Secondary Education with an emphasis in ENGLISH, i think i might know at least that it’s not pm. and you don’t capitalize For! Needless to say, i was alittle mad at his choice of words, so i decided to come home and research. At least tomorrow i can come in and say (from what I’ve gathered here) that mostly it’s PM or p.m. and not pm. Ha ha. Sometimes, ya gotta give it to the man!

It is correct and proper in English to write A.M. or P.M. and not any variation thereof. However, people shorten everything—guess it’s just an attempt to find an easier, less thought out method.