From what I could find, there are theories that perhaps they counted
years in shorter durations than the current calendar, but some point to
similar lengths of months, though maybe not as many months per year.
Does anyone really know?
The issue with this theory is that people are mentioned having children around 50-100, meaning by that count they would be between 4-9 years old.
It is claimed that some of these old time
counters did not grasp the "0" concept, well here they added one, 96.9
years is not far fetched, what is a zero among friends anyway?,
Same problem exists for this explanation, 5-10-year-olds aren't exactly the right age to father their firstborn.
My personal theory is that it was all made based on the Mesopotamian myth that men lived longer before the global flood that is recorded the epic of Gilgamesh.
Back in the day we were taught that man's
longevity was due to the canopy of water around the Earth which filtered
out harmful cosmic rays, which is what eventually kills us.
This
bubble of water is what fell during the flood. The protection was gone.
Immediately after the flood, men's life span reduced dramatically.
I think maybe the dinosaurs died in the flood too.
That canopy of water would actually have been more hostile to life on earth, in fact, that is included in an informative video by Aron-Ra on the meteorological implications of the Noachian Flood.