Hi how are you? Well, this is very simple because at one point the church and government officials were together or as one. While the church put up the AD calendar the governments block the concept of the birth of Christ in any other way. This is the way of religion and secular governments worked as they (governed) the world together, Revelation Chapter 13. In the Bible it tells of how this relationship was made and became very strong and then a sudden changed, Revelation Chapter 18. At the Second Coming of Christ Babylon the Great will be completely destroyed.
Revelation 17:15-18
Living Bible
15 “The oceans, lakes, and rivers that the woman is sitting on represent masses of people of every race and nation.
16 “The scarlet animal and his ten horns—which represent ten kings who will reign with him—all hate the woman, and will attack her and leave her naked and ravaged by fire. 17 For God will put a plan into their minds, a plan that will carry out his purposes: They will mutually agree to give their authority to the scarlet animal so that the words of God will be fulfilled. 18 And this woman you saw in your vision represents the great city that rules over the kings of the earth.”
"Babylon The Great, Mother of the whores and the disgusting things of the earth"
~ Revelation 17:5
"For this reason her plagues of death, misery, and starvation will come in a single day."
~ Revelation 18:8
