Why did Christian countries start counting their years from when Jesus Christ was born?

Answer 1

During the early centuries of Christianity, the traditional Roman means of counting years was used. Originally, years were counted from the ascention of the current emperor, but emperors after the time of Tiberius continued to use the year of his ascension rather than their own.

The Roman abbot Dionysus Exiguus devised the new Christian calendar in 533, in order to remove this honour from a despised pagan ruler. He knew that it was impossible to say when Jesus was born, but he knew, or thought he knew, when Herod died. So, he chose to begin his Christian calendar on the year of Herod's death, and he based this on the reign of the Roman emperor Augustus. Unaware that Augustus only adopted that name four years after his reign began, going by his birth name of Octavius until then, Exiguus commenced his calendar just 4 years too late. However, Exiguus' calendar was largely ignored until Bede (672/3-735) popularised it.

So, the Christian calendar was intended, not to begin on the year of Jesus' birth, but to begin on the year of King Herod's death. The calendar actually begins just four years after Herod died.