but, on the 12 hour clock, "AM" after the hour indicates from 12 midnight to just before noon. (AM is Latin for ante-meridian -- literally translating as something like "before the sun reaches the halfway point, as in "before noon".)

PM stands for post-meridian, or "after noon".

So basically AM is morning and PM is afternoon. The switch from AM to PM comes as the cf clock goes from 11 to 12. So 11AM becomes 12PM. That means that 12AM is actually midnight and 12PM is actually day.

Summary: am means before noon, pm means after noon, exception for your purposes being that 12PM is noon.