Im still unclear how they determined the constant to convert from long mesoamerican to GMT. What common reference event could allow syncing these calendars to a +/- 3 day precision? I would guess some solar eclipse pattern visible from both sides of the Atlantic?
They knew about and could identify solstices, which gives you day of the year. So then it’s just a matter of matching years, which can be done on the basis of things like comets.
Supernovae could also play a factor. Or using tree rings to identify years mentioned as having droughts or floods.
Probably a bunch of other things we haven’t thought of.
