This is not a calendar. It was not created by this Mayan people, but by this much older Olmec civilization, The Maya became the heirs.
It was the Olmecs who shaped the local civilization and culture, not the Mayans as we are told.
The Stone of the Sun (Piedra del Sól) is not a calendar. It is essentially a timer that counts down (Time backwards).
Application:
It consisted of two series of signs. The first one included 20 day signs, which were combined with numbers from 1 to 13. In this way, the tzolkin, i.e. a 260-day ritual timer, was obtained.
The second series consisted of 18 months, each of which had 20 days and was supplemented with an additional sign specifying 5 days (this made it possible to determine the seasons). The whole year, i.e. haab, had 365 days.
The timer counted down the time of the great cycles, of which there are 4. We are currently living in the 4th cycle, which is the last one.
The Great Cycle (also known as the Long Count) lasted approximately 5,126 years and consisted of 13 baktuns, or time units of 144,000 days.