My theory is that someone did some calculations, figured out that in 2012 the would would cross the galactic plane and thought that would be a good STARTING point, and then worked the math and their calendar backwards.
how would they know when we crossed the galactic plane thousands of years ago?
It’s not a good theory because it does not cross the galactic plane in 2012. We’re light years away and it will take millions of years to get there unless we get a huge push so I guess the huge push will be what kills us, not the crossing of the “galactic plane”.