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
Papyrus -Egypt
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
4050 Crusaders
4350 Renaissance
4480 Discovery of the New World
4600 Modern Age
Kepler,
Newton,
Napoleon,
American Independence and French Revolution
Telegraph, Morse Code (4840)
Radio 4890
4900 World Civilisation = 1900
Fax photos
Media Explosion (4920)
ENIAC contract signed 4943
ENIAC 4946
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.