AM and PM

Also, there are at least two acceptable ways to write a.m. and p.m., which are abbreviations for ante meridiem and post meridiem. Ante meridiem is Latin for "before noon" and post meridiem is Latin for "after noon." Note that it is meridieM, with an m, not meridiaN, with an n.

You can write a.m. and p.m. as lowercase letters with periods after them or as small capitals without periods. Either way, there should be a space between the time and the a.m. or p.m. that follows. Although small capitals used to be preferred, it's now more common to see lowercase letters followed by periods (a.m. and p.m.). I suspect this is because it’s so hard to make small caps on a computer.

This is quoted from my favorite podcast, Grammar Girl.