It’s easy to overlook how recent our calendar system is.  Our Gregorian calendar is defined by a number of features: that this is the year 2011, that we use a solar calendar of 12 months, “30 days hath September” and all that, that the year starts roughly 10 days after the winter solstice, the calculation of the leap year, and so on.  Developed during the reign of Pope Gregory XIII, it was introduced into Roman Catholic regions beginning in 1582, but it wasn’t adopted by the British Empire (including America) until 1752, and it wasn’t the world’s predominant calendar system until China adopted it in 1949.
Year 1 must be fixed to some point in history, and myriad dates have been used (and are still being used).  Dionysius Exiguus (Dennis the Short) in the 6th century used the birth of Jesus as the starting point, and this has been the custom in the West since.  Unfortunately, Dennis was off by a few years, and Jesus is now thought to have been born 4–6 years before year 1.
So how do we label years 1 and following?  The Anno Domini (year of our lord) label for this era gradually came into vogue centuries after Dennis, and BC (Before Christ, for the years before) came in later still.
International standard ISO 8601 specifies date and time representations, but it uses plus and minus signs instead of BC and AD.  Unlike conventional dating, it doesn’t bypass the year 0.  Year 10 AD is written as 0010 (4 digits are always used for the year), and year 10 BC becomes –0009 (because of the addition of year 0).
The convention that has become widespread is the use of CE (Common Era) to replace Anno Domini and BCE (Before Common Era) to replace Before Christ.  “Common Era” has been used in English in this sense for over 300 years.  This convention is seen as a way to eliminate outdated religious baggage from the calendar, though there are objections.  Indeed, it was opposition to this convention that prompted the formation of the Conservapedia wiki.
Photo credit: Wikimedia
Related posts:

• See all the definitions in the Cross Examined Glossary.