You would think finding this information would be relatively simple, straight-forward and easy, given the amount of technology at our fingertips. However, I've been searching for nearly 15 minutes on this, which is really too much time, and I'm still frustrated. I have the St. Martin's Guide to Library and Research Documenting, which lists AMA, APA, CBE, and Note-and-Bibliography styles for just about everything, but it also lists abbreviations for time, acronyms, geographical names, and a few other things that are shared amongst all styles. The proper format for time is lower case with periods. It also explains that . is ante meridiem, "before noon" and . is post meridiem, "after noon". This makes sense as to why the periods are necessary, though I don't know how consistent it is across all Latin translations like that. For example, "for example" (<--haha!) we use ".", but for "and others" we use "et al." and not "et. al."