Chicago Manual Style - Time
Am I the only one that gets confused by 12 a.m. and 12 p.m.? Probably not, which is why the Chicago Manual Style suggests always using the terms noon and midnight to avoid misunderstandings (CMS 9.39). That will make my life much easier. The book has a lot of helpful hints dealing with time.
There are so many ways to write the time of day. When using the word o’clock, always spell out the number. “I need to be there by three o’clock.”
Also spell it out if you are using half, or quarter hours. “The meeting begins at one thirty. He got there at a quarter to two.” Although it is not incorrect to use numerals if you want to emphasize the time. “The meeting begins at 1:30 p.m.” (CMS 9.38) Notice you don’t say “six thirty o’clock.” That just sounds wrong.
After spending 28 years in the Air Force, a twenty-four-hour clock comes second nature to me. You start at midnight and count up. Midnight can be either 0000 or 24000, so there is a little flexibility. Once you hit noon, instead of resetting, just keep going. One o’clock is 1300. Four thirty is 1630. It should go without saying, but I’ll say it anyway, that eliminates any need for a.m. and p.m. When you use the twenty-four-hour clock, you should use four digits and no punctuation in between (CMS 9.40). The only time you use colons is if you are adding seconds, 22:03:27= 27 seconds after 10:03 p.m.
Here’s one that I find difficult because it seems unnecessary to me, but it’s most appropriate to use the periods with a.m. and p.m. (CMS 9.38).
Please let me know if there is a style “rule” that is niggling at you that you want help solving. With over 1,000 pages, we can probably figure it out.