The sun shines through the door of the Seven Dolls Temple, in the Maya ruins of Dzibilchaltun in Mexico.

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Long Count Calendar 101

• The Long Count calendar was one of several created by the ancient Maya. 

• It consists of the following units of time: 

kin = one day 
uinal = 20 days 
tun = 360 days (18 uinal) 
katun = 7,200 days (20 tun) 
baktun = 144,000 days (20 katun) 

• The calendar shows the number of days elapsed since the beginning date: August 13, 3114 B.C. (some scholars think the date is actually August 11, 3114 B.C.)

• The dates are written as numbers separated by periods in the following order: 

(baktun).(katun).(tun).(uinal).(kin) 

• July 20, 1969 — the date of the first moon landing — would be written as: 12.17.15.17.0 

• December 21, 2012, would be written as 13.0.0.0.0 and the day after that as 0.0.0.0.1

Source: Howstuffworks.com

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Just as “Y2K” and its batch of predictions about the year 2000 have become a distant memory, here comes “Twenty-twelve.”

Fueled by a crop of books, Web sites with countdown clocks, and claims about ancient timekeepers, interest is growing in what some see as the dawn of a new era, and others as an expiration date for Earth: December 21, 2012.

The date marks the end of a 5,126-year cycle on the Long Count calendar developed by the Maya, the ancient civilization known for its advanced understanding of astronomy and for the great cities it left behind in Mexico and Central America.

(Some scholars believe the cycle ends a bit later — on December 23, 2012.)

Speculation in some circles about whether the Maya chose this particular time because they thought something ominous would happen has sparked a number of doomsday theories.

The hype also has mainstream Maya scholars shaking their heads.

“There’s going to be a whole generation of people who, when they think of the Maya, think of 2012, and to me that’s just criminal,” said David Stuart, director of the Mesoamerica Center at the University of Texas at Austin.

“There is no serious scholar who puts any stock in the idea that the Maya said anything meaningful about 2012.”

But take the fact that December 21, 2012, coincides with the winter solstice, add claims the Maya picked the time period because it also marks an alignment of the sun with the center of the Milky Way galaxy, and you have the makings of an online sensation.

Type “2012″ into an Internet search engine and you’ll find survival guides, survival schools, predictions and “official stuff” to wear, including T-shirts with slogans such as “2012 The End” and “Doomsday 2012.”

Theories about what might happen range from solar storms triggering volcano eruptions to a polar reversal that will make the Earth spin in the opposite direction.

If you think all of this would make a great sci-fi disaster movie, Hollywood is already one step ahead.

“2012,” a special-effects flick starring John Cusack and directed by Roland Emmerich, of “The Day After Tomorrow” fame, is scheduled to be released this fall. The trailer shows a monk running to a bell tower on a mountaintop to sound the alarm as a huge wall of water washes over what appear to be the peaks of the Himalayas…

Read More on CNN.com: Apocalypse in 2012? Date spawns theories, film