Well the history in onepiece world doesn't join. They were dates between 800 AOS-700 AOS (Age of Sea) as appeared in Robin's flashback. People like Cobra & many others know that 100 years timeline is not available but not all of them knew that poneglyphs are recorded history of that time. World govt has prohibited the study of the Void Century. People of ohara were eliminated using Buster Call as an example to the world people to forbid them from reading about the void century. World Govt has hidden themselves behind the excuse that powerful weapon of mass destruction are revived by reading about Void Century and Poneglyphs. Clover has identified a old kingdom existed which was brought down by 20 kingdoms now the World Govt and that ideals of that great kingdom are in direct opposition to