The first and most popular option is to use lowercase "a.m." and "p.m." This method necessitates the use of periods, and both Chicago Style and AP Style suggest it. Every day at 10:05 a.m., this metro train will depart. After ten o'clock p.m. I definitely need to get some rest. Using periods correctly is important because they separate words by adding clarity and understanding the time period in which you are speaking.
The other option is to use capital letters for the term "P.M." and lowercase letters for "A.M." This method is less common but still acceptable. At 10:05 every morning the same train leaves for downtown Chicago. Ten after six means 6:10. No one calls themselves P.M. On the clock, they're just after six.
There are several more options including using sentences instead of short phrases, or even writing whole paragraphs in italics. But since these are extreme cases, we won't cover them here.
Now that you know the proper way to write a personal message, get out there and have fun!
For example, one would write "He went to bed at 11:00 P.M." and "I woke up at 9:00 A.M."
Another common option is to use capital letters for the hour ("9 A.M.") but not the minute. Thus, "9 A.M." would be written as "0900". Using capital letters for the hour but not the minute can be difficult because then you need to remember whether or not to use periods when writing numbers over 100. For example, one would write "He went to bed at 11:00 P.M." but not "11:00 P.M." because there's no point in writing the minutes if you're using capital letters for the hour.
At times, it may be appropriate to use sentence case (all caps) for the entire time reference. For example, "I AWOKE AT 9:00 AM" would be correct if used as an assertion rather than a question. However, this is not common practice and would likely be interpreted as incorrect by most readers.
AM and PM are written in lowercase letters. There are a few standard methods to write these abbreviations in your writing. The other option is to use capitalized "A.M." and "P.M.", which some consider more formal. Neither style guide indicates which option is preferred or not recommended, so use whichever one you think will be understood by readers who may not be familiar with professional shorthand.
Time-of-day abbreviations (usually lowercase only) are handled in a variety of fashions, including "a.m." and "p.m." with a space between the time and the abbreviation ("1.45 p.m."); "am" and "pm" with a space ("1.45 pm"-recognised as an alternate usage by Oxford); and the same without a space...
Abbreviating hours is common in English-speaking countries where people use machines or computers for work; this requires writing time stamps on emails and documents. In such contexts, 12:00 noon and 6:00 pm are typical values for the AM/PM field. However there are no restrictions on what can go in this field - it can also be three-letter acronyms, symbols from various professions, etc.
The United States uses the AM/PM notation in business communications but often omits it in personal letters. Some Americans, especially native speakers from northern states, find this omission odd because they are used to having to write "AM" or "PM" themselves when sending time-sensitive messages like email or making phone calls.
In Australia, New Zealand and South Africa, people usually leave the AM/PM field blank. When these countries were part of Britain, its laws required businesses to print their opening hours on their advertisements, so many publications included a box for customers to write in times they could reach the shop or office.