OK- so "crazy" may be a bit of a mischaracterization- People who lived before geology and biology and paleontoloy were developed to the level that they were in the 1800's- is probably better. Most of the scientists in the 1800's chuckled at the 4000 BC date- because they had found fossils and sediment layers, etc that were older than that.
Its interesting that 7 is the number of things that a person can easily remember (if its a field of interest- if not- usually only 2 or 3) How many state capitals, canadian cities, brands of tvs, etc can a person name- hard limit usually at 7. So marketers remember 7 as a magic number. I gotta believe the bible's master marketers figured that out at some point.