The Goodman-Martinez-Thompson's correlation constant is the JDN of its start date, 13.0.0.0.0.
After cross-checking several Maya colonial documents the GMT constant was determined and it is 584,283. That is, add 584,283 days to 1st of January 4713 BC (Julian) and you arrive at 6th September 3114 BC (Julian), which correlates to 11th August 3114 BC (Gregorian), the calendar in use today.
This correlation leads one to 21st December 2012 for the next 13.0.0.0.0.
