When Christ was born, time was reckoned in the Roman Empire from the founding of the city of Rome. When Christianity became the universal religion over what had been the Roman world, a monk name Dionysius Exiguus, at the request of the Emperor Justinian, made a calendar (526 AD), reckoning time from the birth of Christ, to supersede the Roman calendar.
Long after the Christian calendar had replaced the Roman calendar; it was found that Dionysius had made a mistake in placing the birth of Christ in 753 from the founding of Rome. It should have been about 749, or a year or two earlier.
The Bible does not give the exact time Jesus was born, but information contained in the scriptures does allow us to make an educated guess regarding the year:
Roman historical records indicate that Herod the Great, the one who tried to kill Jesus as a baby, died in the early spring of 750 (Roman calendar). Jesus was born six months to a year earlier—thus in 749 (four years earlier than Dionysius Exiguus had figured).
Luke said that John the Baptist began his ministry “in the fifteenth year of the reign of Tiberius Caesar” (Luke 3:1). Since we know from Roman historical records this reign began in 765, the 15th year was about 780, placing John’s birth no later than 750 and Jesus’ birth six months later.
During the first Passover of Jesus’ public ministry, it was stated that men had started building Herod’s temple 46 years earlier (John 2:20). Available records indicated that Herod began the project in 735, which would place the events of John 2 in 781. Assuming that this first Passover came in the 31st year of Jesus life, this would put His birth about 750.
Taking all of this into consideration, we may be fairly comfortable saying that Jesus was born about 750 or 749 by the Roman calendar (not 753 according to Dionysius). In other words, 4 or 5 B.C.