[Turtle91]

I too prefer the A.M. P.M., but there isn't a requirement to use them as JSWolf implies.

There are plenty of examples that show you can take the first letters from words and make them a new word (acronyms) like: laser and scuba.

There are other ways of shortening a word/phrase: initialisms, and abbreviations.
Initialisms just keep the first letters capitalized but can't say them as a word and are usually not punctuated like: FBI, CIA, USA, AMC, USAF, DOD
Abbreviations are a shortening of the word with a period to show they have been shortened: etc. eg. Abbreviations are not necessarily acronyms.

There is no specific RULE about including the periods in an initialism..., and remember, style guides are just that, guides. The only time it is required to use a specific guide is when your teacher/instructor/professor wants to have something specific to grade you on instead of being subjective.


The moral of the story: Use the method that you prefer - everyone will know exactly what you mean:

9:00 am
9:00 AM
9:00 AM
9:00 A.M.
0900

2:00 pm
2:00 PM
2:00 PM
2:00 P.M.
1400