This is one of those secrets that authors don't explain and keep their readers guessing.
I have never understood the decision to reverse the dates before Christ. Why not simply use the Jewish Calendar or I use my method viz. count the dates from the invention of writing.
1st Millennium 1 == 3001 BC
Clay used for writing - Samaria
2nd Millennium1001 == 2001 BC
3nd Millennium2001 == 1001 BC
4th Millennium3001 == 1 AD
3900 Paper replaces Papyrus (Isn't it amazing that papyrus was used for approx 4000 of the 5000 years of writing history? That deserves a monument or something.)
5th Millennium 4001 == 1001
4480 Discovery of the New World
4600 Modern Age
American Independence and French Revolution
Telegraph, Morse Code (4840)
4900 World Civilisation = 1900
Media Explosion (4920)
ENIAC contract signed 4943
Signals reach Olympus 4990 (Olympus 100 light years away from Earth - They receive the radio signals 100 years after invention of radio spark transmitters.)
6th Millennium 5001 == 2001
Signals from Olympus reach Earth 4990+100+11=5101
(100 light years + 11 years to invent a radio transmitter)
The reason for change in the calendar is one of the anomalies explained at the end of the Adam's Clay story.