In A.D. 525 Dionysius Exiguus, a Catholic monk, was commissioned by Pope St. John I, to determine the correct date for Easter. As a result Dionysius invented the B.C./A.D. time division.
Christ’s birth was used as the starting point to count years before or after Jesus was born. For example, the year 532 B.C. refers to 532 years before Christ was assumed to have been born. A.D. is an abbreviation for the Medieval Latin “Anno Domini,” which means “in the year of our Lord.” B.C. is an abbreviation for “Before Christ. ” Traditionally the A.D. goes before the year, but increasingly it is being used after, i.e. 525 A.D. and the full stop marks are being omitted, e.g. 2011 AD, is increasingly regarded as acceptable.
Exiguus’s counting method determined Christ’s birth to be the year A.D. 1. However more recent data suggests based on evidence relating to Herod’s life and astronomical evidence relating to eclipses and star novae, have led to the conclusion that Jesus was actually born a few years earlier, approximately 4 B.C.
Please note there is no AD 0, which is why calendar purists insist the 21st century actually began 1 January 2001. Dionysius calendar took about 400 years to reach common usage and was in widespread usage after AD 800.
In combination with the 365 day Julian Calendar system (from Julius Caesar) which determined the beginning of months and years and had been in use since 45 BC, Dionysius calendar amendments continued until 1582 AD, when the need for a new calendar became obvious. Because a year is actually 365 and a quarter days long, then every 4 years in the 365 day Julian calendar, March 1st moved behind a day until after 15 centuries, the 1st March was now the beginning of winter, when it had started as the beginning of spring. The result of adjusting to cope with the extra quarter day was the Gregorian Calendar which was introduced in Catholic Europe in 1582 A.D. by Pope Gregory XIII. It definitely has kept the average length of the calendar year more in line with the seasons.
The use of Gregorian calendar as the most widely used calendar in the world today along with the widespread use of BC and AD system came about due to the colonization practices of Christianised Europe and economic pressures of a global economy led by Europe and the USA. As anti- Christian and anti-colonial forces mount they are also seeking to remove the influence of this Christ based BC and AD time system and replace it with the terms BCE and ACE, meaning Before and After the Common Era.