Regardless of which calender they switched to, they nevertheless plotted a 500 year span. Otherwise, if the world was to continue as is, they would have used a 1,000 year or 2,000 year, etc. model. But here's the REAL confusion:
They meant not the end of the world, but the end of the world as WE know it - and we've always only known it with multiple timelines. But after 12/22/12 (or close to it), said timelines will all converge into ONE.
We'll still be here, but no more screwups will be allowed, since we'll never be able to go back in time to fix the future anymore. So whatever we do, we'd better do very carefully, because there will be no makeups or fixes. Everything - no matter how vital or trivial - will now be forever cast in stone!
But hey, no pressure...