The "Tortuguero Monument 6" stone tablet is the ONLY artifact that refers to the December 21, 2012 date. The Aztec Sun Stone does contain a calendar but it was an adaptation of the Mayan calendar and consisted of a 365-day agricultural calendar along with a 260-day sacred calendar.
The "Tortuguero Monument 6" stone tablet was discovered in the 1960's at the archaeological site of Tortuguero in the Gulf of Mexico state of Tabasco that describes the return of a Mayan god at the end of a 13th period. Mexico's archaeology institute acknowledged a second reference to Dec. 21, 2012, on a brick found at other ruins. The Mayan Calendar does NOT end on December 21, 2012 as that is simply the date that the NEXT long-count time cycle BEGINS.