It just hit me that there must have been a start to that standard and it probably wasn't done by Jesus' disciples since they weren't such great historians themselves.
The first account of setting AD and BC dates to the calendar occurred in the 6th century by Dionysius Exiguus, but the system didn't really gain traction until the 8th century, when it was used by Saint Bede in his book Ecclesiastical History of the English People.
Prior to this, the usual method of counting years varied from region to region, but largely it followed the reigns of kings or emperors (the Bible records Jesus' ministry as occuring during "the fifteenth year of the reign of Tiberius Caesar"); Much of the Christian world used Anno Mundi (Year of the World) based on biblical chronology, while the Greeks would count the Olympiads, which occurred every four years.
Anno Domini (Year of our Lord) year 1 marks the birth of Jesus. The year before that is 1 BC (Before Christ). There is no "year zero".
The correct order of writing is the year first for BC (200 BC) and the year last for AD (AD 200).
An interesting side note is that due to incomplete historical records, AD 1 is actually six or so years after Jesus' birth, which occurred around 6 BC. Later on, historical records showed that King Herod as described in the Bible died 5 or 6 years before historians thought he did, but by that time the dates were set.