From the Wikipedia article on Tres Zapotes, a Mesoamerican archaeological site located in the south-central Gulf Lowlands of Mexico.
In 1939, archaeologist Matthew Stirling discovered at Tres Zapotes the bottom half of Stela C. This stela was carved from basalt, with one side showing an Olmec-style engraving that has been variously characterized as an abstract were-jaguar or a ruler on a throne. On other side was the oldest Mesoamerican Long Count calendar date yet unearthed. This date, 188.8.131.52.18, correlates in our present-day calendar to September 3, 32 BCE, although there was some controversy over the missing baktun, the first digit, which Marion Stirling, Matthew's wife, had contended was a '7'. Her judgment was validated in 1969 when the top half of the stela was found.
Since 1939, only one older long-count date has been discovered, Stela 2 from Chiapa de Corzo, Chiapas, with a date of 184.108.40.206.13 (36 BCE).
The back of Stela C is engraved with one of the few surviving examples of Epi-Olmec script. A 1965 study concluded that Stela C, unlike most other basalt stonework at Tres Zapotes, was similar to the basalt used for La Venta Stela 3 and the basalt columns surrounding La Venta Complex A, which themselves have been traced to Punta Roca Partida, on the Gulf Coast at the northern side of the Los Tuxtlas Mountains.