a lot longer than that
The gregorian calendar - and others, Julian, Hebrew, Chinese, Islamic, and the calendars that are no longer followed like the Mayan calendar are all based on lunar cycles, the rotation of earth and earths journey around the sun. These were worked out thousands of years ago ( and probably much longer ) to pretty exacting degree. The calendars that we use now were all based on those ancient sitings and are just basically mathematical organizing of the data. We tend to think that those ancients were not as intelligent as we are, but in reality they had the same deductive brain power, we just have the luxury of knowledge that has been added and compounded throughout the millenia. I have a fantasy of being back in those times just sitting on a knoll on a warm summer night on a hilly plain somewhere looking at the stars ( with no light pollution )and trying to figure out what is what.....and then a sabertooth tiger attacks, or I die from some easily fixed festering infection - something to be said for that compounded knowledge.
