"The Christians Era should properly begin with the year Christ was born; and in devising it, the intention was to have it begin with that year. By the "Christian Era" is meant that system upon which calendars are constructed, and by which historical events are now dated in practically all the civilized world. But the originator of the system made a miscalculation as to the year (in the calendar then in use) in which Christ was born, as the result of which the year A.D. 1 was fixed four years too late. In other words, the Lord Jesus was four years old in the year A.D. 1. [This explains the grave error and colossal blunder of Christians who mistakenly claim that Jesus was crucified in A.D. 33, despite all the evidence. So today's correct year based on this is not 2024 but 2020!]
"The mistake came about in this way: The Christian Era (i.e. the scheme of dates beginning A.D. 1) was not devised utnil A.D. 532. Its inventor, or contriver, was a monk named Dionysius Exiguus. At that time the system of dates in common use began from the era of the emperor Diocletian, A.D. 284. Exiguus was not willing to connect his system of dates with the name of that infamous tyrant and persecutor. So he conceived the idea of connecting his system with, and dating all its events from, the Incarnation of Jesus Christ. His reason for wishing to do this was, as he wrote to Bishop Petronius, "to the end that the commencement of our hope might be better known to us, and that the cause of man's restoration, namely, our Redeemer's passion, might appear with clearer evidence."
"For the carrying out of this excellent plan, it was necessary to fix the date of the Incarnation in the terms of the chronological systems then in vogue. The Romans dated the beginning of their history from the supposed date of the founding of the city (ab urbe condita or A.U.C. as usually abbreviated). Dionysius Exiguus calculated that the year of our Lord's birth was A.U.C. 753. He made his equivalence of dates from Luke 3:1, "now in the fifteenth year of the reign of Tiberius Caesar" etc., at which time Christ was 30 years of age according to Luke 3:23. But it was ascertained later that a mistake of four years had been made; for it clearly appears by Matthew 2 that Christ was born before the death of Herod, who died in 749 A.U.C. Tiberius succeeded Augustus, Aug. 19, A.U.C. 767. Hence his 15th year would be A.U.C. 779; and from those facts Dionysius was right in his calculation. But it was discovered in later years that Tiberius began to reign as colleague with Augustus four years before the latter died. Hence the 15th year mentioned by Luke was four years earlier than was supposed by Dionysius, and consequently the birth of Christ was that many years earlier than the date selected by Exiguus, which date has been followed ever since. This must be allowed for in any computation of dates which involves events happening before Christ.
"We have now found, according to our reckoning, that Christ was born An. Hom. 4041. Therefore, His crucifixion, when He was in His 34th year, would be 4041+34=4075. This is equivalent to 30 A.D.; and to get the true measure of years of any event in our era from the Incarnation it is necessary to add four years to its accepted date. [That would put us at ~6069 A.H. (Anno Hominis, year of man or year of humanity).]
"To get the corresponding date in terms of B.C. for any event of Old Testament history, it is only necessary to deduct the years An. Hom. from 4046. For the birth of Christ being 4041 An. Hom. and the Christian era four years later, then B.C. 1 would be equivalent to 4045 which is 4046-1."
—Philip Mauro, The Wonder of Bible Chronology