On September 6, 3114 BC, the date the Maya/Mesoamerican Long Count Calendar started dating from (as corresponds to the Julian Calendar). From the article:
"The Mayan Calendar
How Does the Mayan Calendar Work?
The Maya calendar is a system of three interlacing calendars and almanacs which was used by several cultures in Central America, most famously the Maya civilization.
The 2012 Phenomenon
The Mayan calendar is an ancient calendar system that rose to fame in 2012, when a “Great Cycle” of its Long Count component came to an end, inspiring some to believe that the world would end at 11:11 UTC on December 21, 2012. The media hype and hysteria that ensued was later termed the 2012 phenomenon.
Of course, the predictions did not come true—just like hundreds of other doomsday prophecies that fizzled out in the past.
Not a Maya Invention
The Mayan calendar dates back to at least the 5th century BCE and it is still in use in some Mayan communities today. However, even though the Mayans contributed to the further development of the calendar, they did not actually invent it. The same system was used by most cultures in pre-Columbian Central America—including those predating the Maya.
Wheels Working Together
The Mayan Calendar consists of three separate corresponding calendars: the Long Count, the Tzolkin (divine calendar), and the Haab (civil calendar). Each of them is cyclical, meaning that a certain number of days must occur before a new cycle can begin.
The three calendars are used simultaneously. The Tzolkin and the Haab identify the days, but not the years. The Long Count date comes first, then the Tzolkin date, and last the Haab date. A typical Mayan date would read: 13.0.0.0.0 4 Ahau 8 Kumku, where 13.0.0.0.0 is the Long Count date, 4 Ahau is the Tzolkin date, and 8 Kumku is the Haab date.
The Haab
The Haab is a 365-day solar calendar which is divided into 18 months of 20 days each and one month which is only 5 days long (Uayeb). The calendar has an outer ring of Mayan glyphs (pictures) which represent each of the 19 months. Each day is represented by a number in the month followed by the name of the month. Each glyph represents a personality associated with the month.
The Haab is somewhat inaccurate as it is exactly 365 days long. An actual tropical or solar year, the time it takes Earth to orbit the Sun, takes about 365.24219 days on average. In today’s Gregorian calendar, we adjust for this discrepancy by making almost every fourth year a leap year, when an extra day—a leap day—is added on the 29th of February.
The Tzolkin
The Tzolkin, meaning “the distribution of the days,” is also called the Divine Calendar and the Sacred Round. It is a 260-day calendar with 20 periods of 13 days, and it is used to determine the time of religious and ceremonial events. The days in each period are numbered from one to 13. Each day is also given a name (glyph) from a sequence of 20 day names.
The Long Count
The Long Count is an astronomical calendar which is used to track longer periods of time. The Maya called it the “universal cycle.” Each such cycle is calculated to be 2,880,000 days long (about 7885 solar years). The Mayans believed that the universe is destroyed and then recreated at the start of each universal cycle. This belief caused the 2012 phenomenon described above, and it still inspires a myriad of prophecies about the end of the world.
The “creation date” for the current cycle is 4 Ahau, 8 Kumku. According to the most common conversion, this date is equivalent to August 11, 3114 BCE in the Gregorian calendar and September 6, 3114 BCE in the Julian calendar.
How to Set the Date
A date in the Mayan calendar is specified by its position in both the Tzolkin and the Haab calendars. This creates a total of 18,980 unique date combinations, which are used to identify each day within a cycle lasting about 52 years. This period is called the Calendar Round.
In practice, the date combinations are represented by two wheels rotating in different directions. The smallest wheel consists of 260 teeth with each one having the name of the days of the Tzolkin. The larger wheel consists of 365 teeth and has the name of each of the positions of the Haab year. As both wheels rotate, the name of the Tzolkin day corresponds to each Haab position.
The date is further identified by counting the number of days from the “creation date”, using the Long Count calendar. A typical Long Count date has the following format: Baktun.Katun.Tun.Uinal.Kin.
Kin = 1 Day.
Uinal = 20 kin = 20 days.
Tun = 18 uinal = 360 days.
Katun = 20 tun = 360 uinal = 7,200 days.
Baktun = 20 katun = 400 tun = 7,200 uinal = 144,000 days.
The kin, tun, and katun are numbered from zero to 19; the uinal are numbered from zero to 17; and the baktun are numbered from one to 13. The Long Count has a cycle of 13 baktuns, which will be completed 1.872.000 days (13 baktuns) after 0.0.0.0.0. This period equals 5125.36 years and is referred to as the “Great Cycle” of the Long Count.
Mayan Culture Today
The Maya still form sizable populations that include regions encompassing present-day Guatemala, Belize, Honduras, El Salvador, and parts of Mexico. They maintain a distinctive set of traditions and beliefs, which was inspired by a combination of pre-Columbian and post-conquest ideas and cultures."