Although we don't use them often when speaking, the terms "A.M." and "P.M." occur frequently in written text. Although there are several different rules that can be followed when using these designations, the most important point to remember is to be consistent throughout your publication. The following are some guidelines:
Each of these is an acceptable form of use:
A.M. and P.M.
AM and PM
a.m. and p.m. (generally, the most popular form)
am and pm (not used as often)
Take care when using the last type of formatting, as it can make your text less readable. The key point, again, is to be consistent. Mixing formats can make your reader stumble and makes your publication's design look cluttered and unprofessional. Also, there should always be a space between the numbers and the A.M. or P.M. designation.
"A.M." stands for "ante meridiem," which is Latin for before the midday. "P.M." stands for "post meridiem," or after the midday. So any time before noon is designated as "A.M." A time after noon is a "P.M." time. But what about noon and midnight? Technically speaking, one second after noon is just that--after noon. So 12:00 noon can be labeled at 12:00 P.M. Likewise, midnight can be designated as 12:00 A.M. But it is just as acceptable to use "12:00 noon" and "12:00 midnight."
If you are including several times that include before and after noon times, be sure to label each appropriately. For example, this is an acceptable use of these qualifiers: "I had to catch a plane at 1:45 P.M., so I had to leave my house at 11:00 A.M." If you are listing several times, only include the qualifiers for each group. For example, "My day is booked. I have appointments at 9 and 10:30 A.M. and then at 1, 3, and 4:30 P.M."
- It is optional to include the zeros in the time if it is exactly on the hours. For example, either 4 p.m. or 4:00 p.m. is acceptable.