It's AM and PM, not am and pm. Although "ante-meridian" and "post-meridian" are in lower case, you write letters as ABC, not abc. Otherwise you would pronounce "am" as "ahm" and "pm" as "pumh". One o'clock ahm, one o'clock pumh. "AM", on the other hand, is "Ay-Em", and "PM", "Pee-Em".

The same is true with AD, BC, CE, BCE, AC-DC, and others more.