This is where we get our "a.m." and "p.m." from.
10:00am = ten o'clock ante meridiem
10:00pm = ten o'clock post meridiem
It is, however the Romans reckoned time differently than we do. The first hour was dawn, when the sun reached its zenith (noon) was the sixth hour, and sunset was the twelfth. Night was similarly divided.
Yes, that's true, although that's tangential to the etymology of "a.m." and "p.m." per se.