reference from wiseGeek
In one respect, there really is no difference between an AD/BC and BCE/CE system when it comes to historical dates. The year 23 AD is exactly the same as the year 23 CE, and 4004 BC is also 4004 BCE. References to historical dates under either classification shouldn't create confusion in a researcher's mind. Major historical dates such as 1492 AD, 1776 AD or 1941 AD would still be rendered as 1492 CE, 1776 CE and 1941 CE.
The AD/BC method of identifying historical dates can be traced back to Catholic historians working in the early Middle Ages. Identifying historical dates until that point was often a complicated proposition, since different historians worked under different calendars. A Roman historian would have used the Roman AUD notation, in which Year Zero was the largely symbolic founding of Rome. Converting historical dates to the standard Gregorian calendar would not have been easy. Using the birth of Jesus Christ as a central point made more sense to the religious historians.