They did finish the calendar. Their calendar was a lunar calendar, and it cycles till the planets align, every so many thousand years. If you were a calendar maker hundreds of years ago , and ran the calendar out till the end of the cycle, you would of probably stopped at that point also. Put another way....how many of you have calendars up on your wall years in advance? I don't. Our calendar happens to be yearly. And if a alien came to visit and looked at your calendar, how foolish would it be to assume that YOU predicted the end of the world as the last day of December, due to the simple fact YOUR calendar ended? The different civilizations all came to one simple conclusion. NOT that the world would end at that time...just the calendar ended, and they simply had no reason to start the next one. Too much coincidence you say? That would be like that alien finding the same calendar all over the world, and suggesting that due to all people forcasting the same ending in December, that the proof is overwhelming. We all use the yearly calendar. Many years ago, they all used the same lunar calendar. Simple as that.