It is really important to understand where the December 21, 2012 date comes from—as you note it comes from the interdisciplinary work of three scholars. They worked with dates recorded on long count monuments C-14 dated between 300 a.d. and 900 a.d. The correlation of the Mayan calendar and Gregorian calendars is based upon this range, and gives us December 21, 2012 as equivalent to 13.0.0.0.0. In other words, the data comes from a context after 4 b.c. so there is no need to correct for a 3 or 5 year gap of "uncounted" time. More to the point, if for example the Long Count date 9.16.4.4.1 was determined to be equal to May 7, 755 a.d. in the Gregorian calendar, a predictable number of days would have to elapse before 13.0.0.0.0 was reached. That number of days brings us to December 21, 2012. If we had implemented an adjusted system to correlate our own year counting with the actual birth year of Jesus, the same number of days would have to elapse but the date would be called, not December 21, 2012, but December 21, 2017. This seems to be a very roundabout and belabored way of clarifying something that is basically not really an issue. If a researcher proposes that the end-date is "really" in 2017, or some other year, and the argument is based upon an adjusted system to account for the true birth year of Jesus, then that adjusted year must be designated as the "adjusted year count" and if we looked at how it correlates with the old "pre-adjusted" Gregorian calendar, it would correlate with the year 2012. It's not that time is skipped or missing—the entire frame of counting is simply shifted 5 years.