As you already know, there are 12 hours in AM and PM, it's the very correct way of saying the time.
However, we use ごご for "afternoon" in everyday conversation. (Ex. 午前中はいそがしいので、午後一時に電話をします[ごぜんちゅうはいそがしいので、ごごいちじにでんわをします]。= Since I'm busy in the morning, I'll call you at 1:00 in the afternoon.)
We also use あさ(early in the morning)、ごぜん(in the morning)、おひる(at noon / around noon)、ゆうがた(in the evening)、よる(at night) in everyday conversation but at the station, airport, etc, or when time must be said officially, they always use AM(ごぜん) and PM(ごご) for the time of transportation.
I hope this helps you.