Both dates are used.. My Dell Horoscope mag uses the December 23rd, And here is a explanation from Great dreams .com

The canonical dates (according to Ehran the Scribe) on the ends of the Fourth, Fifth, and Sixth worlds are August 12, 3113 BCE (the fall of Atlantis), December 24, 2011 CE (the appearance of Ryumyo), and April 4, 7137 CE. This gives 5123-5125 years per mana cycle [1,871,270 days for the Fifth World’s duration, and 1,871,969 for the Sixth World’s]. The Mayan Long Count suggests 1.872M days, or 5125.36 years. It also seems that Ehran’s dates are a year late (0.0.0.0.0 should be August 11 or 12, 3114 BCE— the astronomical year –3113) and a year early (13.0.0.0.0 corresponds to December 21 or 23, 2012).

[link to www.greatdreams.com]