Aztec calendar correlation problem?...

Hi,

I have a doubt regarding the correlation dates for the Aztec Calendar.
It's widely accepted that the fall of Tenochtitlan was in August 13, 1521 - which corresponds to the Julian date number 2276828 (at 12AM).
This date is correlated to the aztec date 2 Xocotlhuetzi 1 Coatl.
On the other hand, another well knowed date is refered: the entrance of Cortez in the city, in November 8, 1519.
This date corresponds to the Julian day 2276184 (12 AM).
It's refered to the aztec date 8 Ehecatl 9 Quecholli of the year 1 Acatl.
The problem is that if we take the first correspondence, we will get for the second julian day number the date 7 Cipactli, 8 Quecholli.
It's one day of delay.
If we take the second date as reference, the first will give us one day after the citation.
There's always one day of difference - I think I'm not mistaken.
All the sites that do the calculations based on Alfonso Caso correlation, and in the first correlation date I've mentioned, cleary show the discrespancy in the calculations.

Can someone please tell me if I'm missing something here?

Kind regards,

Kepler