We can make many misstakes attempting to understand ancient texts and sign posts, the most erroneous is to assume we know more then ancient peoples, another is to believe we understand what they were attempting to communicate.
In Gary Zukav's book "The Dancing Wu Li Masters" he presents compelling evidence showing that physics has just begun to cognize the Universe in depths that can be found in ancient vedic texts. If you understand the impact of this, you realize that mystics understood Existence or Being long before the invention of science.
The second part of this is, that to understand Vedic or mystical texts it is imperative to know that these texts cannot be understood through thinking. They are not presenting constructs, they are intuitive cognitions, projected through mystical experience from within the field of absolute being. To KNOW, one must experience the same. Without the experience, one can only guess at what the experience is, there is no knowing.
So with that said.
It is comforting to assume that 'time' is constant. Yet time is merely a concept of measurement. An interval, a unit depicting the movement or change of form or matter. Passage or flow of thoughts reinforces the illusion that time is real.
I cannot attempt to explain the Mayan calendar, I can offer an interesting view that may lend support to...whatever may be.
Terence McKenna in his work "Time Wave Zero" cognized and provided proofs for an underlying pattern to the evolution of existence.
Reflecting on a vision that form or matter is vibration within the field of undifferentiated, Mckenna's work may be translated in terms that form or matter vibrates within a field of undifferentiated which also vibrates and does so in a recognizable pattern. This underlying vibration ultimately effects all.
Simply put, this patten can be compared to a music scale and it's octaves. As the Universe evolves it moves from one vibration to the next and then to the next octave, each octave allowing for another level of complexity to come into existence.
And similar to a musical scale there maybe a limit to the range of octaves.
Maybe we are reaching the end of the range or just the limit of our capability to cognize the range.