A.M. and P.M. - A few countries, namely the United States, Canada, and Australia, use the 12-hour clock instead of the 24-hour clock. This results in a need to differentiate the two halves of a 24-hour period (morning versus night), so the abbreviations "a.m. and p.m." were created. The origin of these abbreviations is simple ---"a.m. stands for the Latin phrase ante meridiem, meaning "before noon", while " p.m." is short for post meridiem, the Latin phrase for "after noon." The 24-hour system, which is used throughout the rest of the world, has no need for such abbreviations.   B.C. and A.D. 

are the oldest date abbreviations still in use. "B.C." stands for "before Christ," as in, before the birth of Jesus Christ, a central figure in Christianity. "A.D." stands for anno Domini, meaning "in the year of our Lord" in Latin. There is no year zero in this system---1AD. was the year Christ was born, and 1B.C. was the year before he was born. Although the B.C./A.D. system took a while to become standardized, it smoothed out a lot of calendar confusion. Before this dating system, years were centered around significant events, such as the rules of emperors and kings., leaving room for a lot of misinterpretation and errors. The B.C./A.D. system was the first time that every single year was accounted for in such a uniform way.