Dating events from various proxies, and comparing to calendar design changes and alterations.
Calendars need to be axially aligned to equinox sunrise point on horizon. At ~5200bce and then at ~3200bce there are changes of axial orientation, always CCW, indicating major cataclysmic seismic events. (Note: viewing angle is equinox to solstice of horizon sunrise point, at latitude 35.8 deg N).
Below pic is same with added info for comparison. (30/08/2019)