"What time is it?"

Keeping track of the time is important for most people and when the time is not immediately available, you may be asked (or have to ask someone), “what time is it?”  The response may be, “it is 7 O’clock” and generally you will know if it is 7 am or 7 pm. 

What does am mean?  What does pm mean?  Where do these common terms come from?  The term am means anti meridiem, which means before noon. The term pm means post meridiem, which means after noon.    

According to Wikipedia the reason for using this measurement of time was because the Romans used of a 12 hour clock.

The Romans also used a 12-hour clock: daylight was divided into 12 equal hours (of, thus, varying length throughout the year) and the night was divided into four watches. The Romans numbered the morning hours originally in reverse. For example, "3 a.m." or "3 hours ante meridiem" meant "three hours before noon", compared to the modern usage of "three hours into the first 12-hour period of the day".