I'm not sure that a "flexible" calendar is such a good thing. It seems to be about the same as no real calendar at all; just looking at the sun and deciding that today is a good day to start the new year. The point of a calendar is its rigidity; if one understands the calendar system, then one can figure out the exact date 2000 years ago, and one can figure out the exact date 2000 years in the future. Well "years" is a bad day, but my point is that a calendar should let us say that 2,000,000 days from today, it will be the 20th day of the fifth month of the year 50000 (or whatever). Saying that the date will be 20-5-50000 plus or minus however many days we decide to skip along the way is somewhat contradictory to the utility of a calendar, in my opinion. I know this doesn't help with the changes in Earth's revolution or orbit or whatnot, but then why bother with a calendar at all? We could just name days according to the position of the Sun, determining each as we go along, and use the 7 day week for planning.

Hold on, I think I'm on to something here. :)