The Common Era is the period beginning with a year near the birth of Jesus, coinciding with the period from AD 1 onwards. It is synonymous with the period called the Christian era and is sometimes used as a religiously neutral alternative to it. The term has been in use since the late 19th century and is common in academic circles around the world. Though it is increasingly used, it cannot be said to have entered into general use by the public.
Though there are many calendar systems, the Gregorian calendar has achieved prominence in practical use worldwide. The days, months, and leap years of its direct predecessor, the Julian calendar, were devised by Romans in pre-Christian times. Virtually all Romans identified each year by naming the two consuls who held office that year until the sixth century, resulting almost always in non-numerical year notation.
Early Christian histories counted the years since the beginning of the world. However, in 525, the Anno Domini system was invented, which counted the years of the Julian calendar from the year of Jesus's birth. The transition by the Western Christian church to the Gregorian calendar, which was promulgated in 1582, corrected seasonal errors due to an incorrect leap year system, though this correction left the numbering of the years intact.
The spread of the Gregorian calendar has made it the standard international chronology, though some countries and cultures use their own calendars for religious and cultural purposes.
The Common Era and the modern calendar
When used as a numbering system for years, the Common Era is abbreviated as CE and the BCE/CE system is synonymous with the much more common BC/AD system ("before Christ" and "anno Domini," or "in the year of [our] Lord [Jesus Christ]"). Thus 500 CE is the same year as AD 500. Similarly, "before the Common Era" and BCE are used synonymously with "before Christ" and BC. Just like the AD/CE equivalency, any "BC" year is the same "BCE" year, so it is equally accurate to say that Julius Caesar was assassinated in either 44 BC or 44 BCE.
Both "CE" and "BCE" follow the year. This contrasts with proper use of "AD", which mirrors the fact that the original Latin is a prepositional phrase, so "in the year of the Lord 1601" is correctly written as "AD 1601", and not as "1601 AD", as many people write. This is opposite to correct usage of "BC", which is always written after the year it modifies (e.g. "44 BC").
Usage of term
The Common Era designation is most often used by academics, especially by scholars of non-Christian cultures. Some non-Christians, who believe that the incorporation of Jesus into the international timekeeping standard clashes with their own religious or secular beliefs, have also adopted the designation. Also the New Testament has very little date reference, only indirect ones such as the depiction of persons mentioned in independent historical sources. If the parts of the Gospel depicting the birth of Jesus are correct, he could not actually have been born in AD 1. This is because we know from the historical record that Herod the Great (depicted as threatening the life of the infant Jesus) died in 4 BC. Therefore, most scholars of the New Testament regard Jesus as having been born between 4 and 7 BC, seeing the AD/BC system as an early "best guess" adopted by Christians to give coherence to a new universal religious, cultural and political system.
It is used in interfaith dialogue by some Christian churches (http://www.torontoareamennonites.ca/danforth/dmc_notes/witmer10.htm) and by some news media in the United States (http://seattletimes.nwsource.com/news/nation-world/mideast/roots/) in articles when dealing with Palestine or interfaith matters. Secular organisations that use BCE/CE include the Royal Ontario Museum in Canada (http://www.rom.on.ca/ossuary/ossuary_intro.html), and the U.S. National Center for History in the Schools, which publishes the National Standards for History taught from kindergarten to 12th Grade.  (http://nchs.ucla.edu/standards/world-standards5-12.html).
Although Common Era dating is widespread amongst historians, archaeologists, and other academics, it has not gained general acceptance outside those groups, and the general public is still largely unfamiliar with Common Era notation. In addition, some writers who view "Common Era" as an attempt to remove Christian references from the calendar use "CE" notation as shorthand for "Christian era", a term that predates "Common Era".
Reasons for opposing the Common Era designation include:
- It downplays the prominence of Jesus Christ in majority-Christian societies.
- The months and days of the week, named respectively after Roman and Norse gods, remain unchanged, so attempts to remove references to Jesus in the calendar are hypocritical.
- It is an example of political correctness.
- It preserves a Christian-centric worldview, at the expense of a neutral, non-religious timekeeping system. The year AD 1 is not 'common' to many modern cultures, where it is not the standard dating and where Jesus' birth (and the start of the Christian Era) are not considered major landmarks in the world's history.
- It is not as "common" as some people may believe. For example, most Muslim countries still use the Islamic calendar as their official calendar. Another example would be Thailand, which officially uses the Buddhist calendar.
- Religious Tolerance.org: The use of "CE" and "BCE" to identify dates (http://www.religioustolerance.org/ce.htm)
- Chicago Manual of Style note on usage of CE/AD in the United States (see ninth question) (http://www.press.uchicago.edu/Misc/Chicago/cmosfaq/cmosfaq.Abbreviations.html)
- Whatever happened to B.C. and A.D., and why? (http://www.ucc.org/ucnews/jan03/asiseeit.htm)
- In British schools:
- This is London: Evening Standard, 19 February 2002: AD and BC become CE/BCE (http://www.thisislondon.co.uk/news/articles/531644)
- News.Telegraph: 13 December 2002: History has to be rewritten as school bans BC and AD (http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2002/12/13/ndates13.xml)
- In U.S. schools: