just found out about the speculation1 that the year chosen by dionysus exiguus (the guy who invented the anno domini system in anno domini 525) for the birth of jesus was chosen in part because it implied that the world was currently more than 6,000 years old, thereby dampening the fears of millennarian christians who believed that the world would end in anno mundi 6000.
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all the citations on wikipedia for this idea refer to the book "anno domini: the origins of the christian era" by georges declercq, who I have no idea as to the esteem in which he's held by theologians. I just find the idea that people have been tired of millennarians since the sixth century very, very funny.