"Specifically, let us assume that shana comes from the word repeat
(shny, and not from yshn), and is thus related to the word shnayim,
two. Further, let us assume that in the time in which the initial
genealogical list were recorded, chodesh is a month, and shana is a
two-month period. We could say this even without the etymology. If so,
we would need to divide each "year" by 6 to get the actual year."
Don't know if this is reason enough to re-open the conversation, but I
thought it interesting.
—Joel Salomon
[b-hebrew] Biblical year lengths,
Joel C. Salomon, 10/19/2009