The American expression of time always confuses me, especially when people say "12 am" or "12 pm".  What time is on earth "12 am"?

It took me very long time to figure out the day actually started at "12 am" and the afternoon started at "12 pm".  Because 12:00 is either noon or midnight in Chinese, which means we can not call it "before noon（上午）" or "afternoon（下午）".  Thus the proper way to say is: 中午12点（正午12点 is possible as well） or 午夜12点（子夜12点、半夜12点 are both correct）.   

Since we are talking about the time, let get more details. Check the list below of the most common used time words:

凌晨: from midnight to anytime before dawn, for example we can say 凌晨5点 in winter when it's still dark outside, while it may not be suitable in summer.

早上: early morning, usually it's the time when people have breakfast and go to work or school.

上午: morning, can be used to indicate anytime between dawn to noon. 

中午/正午: noon.

下午/午后: afternoon, noon to dusk.

傍晚: dusk.

晚上: night, usually used for the time between dinner to bedtime.  Since Chinese have dinner at 6-7pm, 晚上 starts about 6pm and end as late as midnight.

夜里: from dusk to dawn, it used to mean anytime when dark outside, but nowadays people say 夜里 mean late night to dawn more, like 8pm or 3am.

半夜/子夜/午夜