In addition to answers of Ryoko-san and Nao-san, I 'd like to say this. 日本語<にほんご> Japanese 正午<しょうごsyoogo>noon＝午前<ごぜんgozen>12時<じゅうにじjuuniji>＝午後<ごごgoo>零時<れいじreeji> 英語<えいご>English noon＝12 P.M. 日本語 午後12時＝午前零時<れいじ> 英語<えいご> 12 A.M. Therefore we(Japanese) must be careful when we promise with American. We might think that the time is "午前12時" when the American say "12 A.M. ".

We use ごご for both ways. As an additional point, when we use the pm in the sentence, the time is just after ごご. In other word, when you find ごご without any following words related to the time, this means afternoon. And, yes! You can say that. I will put an example here. ごぜんははれてましたが、ごごはあめですね。 Thanks,

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.