Latin Meaning

• In Latin, the abbreviation A.M. stands for "ante meridiem," or before midday. The abbreviation P.M. stands for "post meridiem," or after midday.
  
  

Use of A.M. and P.M.

• Knowing the words these letters stand for, the Latin meaning of those words and the fact that the 12-hour clock is split into two 12-hour time periods, it becomes clear that a time with the P.M. designation includes the times following noon until just before midnight, and the time with the A.M. designation includes the times following until noon until midnight.
  
  

Caution

• To be completely clear, it is best to avoid the use of 12 A.M. and 12 P.M. since both are technically incorrect. Instead, use "noon" and "midnight." Time-sensitive industries, such as airlines and railroads, refer to the end of the day as 11:59 P.M. and the beginning of the day as 12:01 A.M.
  
  

24-Hour Time

• The use of 24-hour time avoids the need to use A.M. and P.M. In 24-hour time, there is only one "one o'clock" time, which is one hour after midnight. One o'clock in the afternoon is 13:00, pronounced 13-hundred, hours.