Finally got to take a break to stop and breathe. As you've doubtless figured out by now, the date at the top of that image I left is of particular interest to the Aztec people, while the bottom one seems to have no correlation to any events I can find.
This leads to one of two conclusions.
Conclusion one: The second date is a reference to a future date for the Aztecs, whether it be in our past or future.
Conclusion two: There's an alternate history to what we know about the Aztecs that we either haven't uncovered or has been covered up, and this date is of importance to that history.
It seems to me that currently conclusion two is more likely. What I've found in my notes currently seems to lead to... contradictions with what is known about the Aztec empire and its history. Specifically, there's an exclusion of details. Large portions of history, particularly the ones you would think would be the most important aren't mentioned anywhere. It's entirely possible that those parts were merely missing, but why would JUST those parts be missing?
I'm still fumbling through my notes trying to find a place to start. The first step is always the hardest. When all you have is fragments, it's hard to put the puzzle together, but once a few pieces fall in place, the whole thing cascades.
A curious issue that keeps coming up is the constant reference in my research of a being known only as the Meddling Man. Despite constant mention, it's always in passing and doesn't seem to give too much detail on their purpose. The only thing that's clear is that this person, this being, is important to the Aztec and some plan.
I'll continue through my notes and keep you updated.