Regardless of how inaccurate the Gregorian calendar may be it has no effect on the Mayan calendar. The Mayan calendar is independent of the Gregorian calendar. It just so happens when you convert the end of the current 12th baktun date 13.0.0.0.0 to the Gregorian date it comes out to December 21, 2012. If 5 years of the Gregorian calendar were lost and it is really 2013 then the Mayan date 13.0.0.0.0 would not happen until 2017. Understandable now? It is easy to check the Mayan calendar because it often marks certain celestial alignments.