Now, to answer Corbyloon's question.
Dionysus Exiguus (I believe I have the spelling right but am happy to be corrected) attempted to calculate the date of Christ's birth in the seventh century AD (as we now know it). However he got the date a little wrong (we know it is wrong for this reason, according to biblical sources): Christ must have been born during the reign of Herod the Great, but Herod died in the spring of 4BC so Christ must have been born before then, possibly as much as two years before.
There is an added complication: historians calculate BC dates on the assumption that the year immediately before 1AD was 1BC. However, astronomers call the year before 1AD year 0, because it makes calculations easier. BC dates are simply represented by negative numbers, so the eclipse which occurred at the death of Herod occurred (according to astronomers) in March -3
Astronomers on AB will no doubt tell me that they use Julian days anyway...