The Christian era, now used almost exclusively throughout the Western world for civil chronology, was first used in 525 by the Christian monk Dionysius Exiguus, who fixed the birth of Christ in the year of Rome 753. ... Dionysius's chronology was introduced into historical writings by Bede the Venerable in the 8th century.

"Chronology," Microsoft® Encarta® 97 Encyclopedia. © 1993-1996 Microsoft Corporation.

The correct way to state dates with the above signifiers is as follows:

• The Second World War ended in AD 1945. (This is sometimes spelled out as "in the year of our Lord 1945.")
• The Second World War ended in 1945 CE ("the year 1945 of the Common Era").
• Gaius Julius Caesar was assassinated in 44 BC ("the 44th year before Christ").
• Gaius Julius Caesar was assassinated in 44 BCE ("the 44th year before the Common Era").

It cannot be overemphasized that there is NO YEAR ZERO. The year preceding AD 1 was 1 BC!

If you are referring, not to the system of numbering years but to the system of months which makes up each year, that calendar has no relationship to Christianity, but was introduced by Julius Caesar in about 45 BC. The system of leap years was also part of the Julian Calendar. The last adjustment was the removal of century leap years (except that a century year divisible by 400 - such as 2000 - is still a leap year) in AD 1582 by Pope Gregory XIII; this is called the Gregorian Calendar.

Because it was introduced by Catholics, Protestant countries (notably Britain, and its American colonies) did not all adopt the Gregorian Calendar until AD 1752. The Orthodox Church successfully resisted the adoption of the system in Russia until the Bolsheviks took over in 1918, and while Greece adopted the Gregorian Calendar as her civil calendar in 1923, Eastern Christian Churches still calculate feast days based on the Julian Calendar. This seems to be why Orthodox Easter typically comes a week or so after it is celebrated by Western Christians.