AD (or A.D.) is short for Anno Domini. The term Anno Domini is Latin, meaning In the year of our Lord (Jesus Christ). In British English, some very commonly used abbreviations do not use the full stops. American English usually keeps the full stops. So this is the year AD 2013 in the UK and A.D. 2013 in the US. The letters AD always go before the year number.
Long ago in ancient Europe, counting year to year was done according to the reign of a king. For example: In the twenty-fourth year of his reign King Edward of England invaded Scotland. That made extra work and confusion for historians keeping track between kings. Then each country had its own kings. Ordinary people did not have to worry much about years; planting and harvesting times were what they needed to know. Long ago a boy might say that he was twelve winters old, but there was no name or number for the year of his birth. So a famous historian developed a better system to count years and make it easier to study history.
An English monk called Bede thought that it would be easier if all years were counted from one ancient date, instead of when different kings took power. What date did not matter very much, but it should be something that was both neutral and more important than any king. As a Christian monk, Bede chose the (estimated) year of the birth of Jesus. He created & popularised this system of numbering years. (He probably built on the more limited work of the monk Dionysius Exiguus.) Bede, later called the Venerable Bede, was able to rewrite much of Western history with easy to understand dates.
Some people prefer to rename the system as the Common Era or the Current Era, instead of AD, making this the year 2013 CE (or C.E.). The Venerable Bede's history did not deal with the dates before Jesus; too little pre-Christian history was known. So his system was adapted and dates before the birth of Jesus are noted with an English abbreviation BC (or B.C.) for before Christ. Unlike in the Latin AD notation, the letters go after the number, for example: In 196 BC, Zhao Tuo king of Nanyue paid tribute to the Emperor of China. As with the use of CE instead of AD, some people prefer to use Before Common Era or BCE (or B.C.E.). Other people argue that since it was a Christian monk who created this system, it would be polite to respect his choice of the year to count from.
Through an accident of history, people all over the modern world from many religions, countries and cultures have all ended up using this Christian calendar. The Venerable Bede developed his system in a mostly Christian Europe. The known world is much larger today and the choice of the birth of Jesus is no longer as neutral as it once was. However his choice is not going to change, because it would be too much trouble to get everyone to agree on a replacement.
by John Larrysson
A native English speaker who has been teaching practical English in Hong Kong for more than a decade.