There was a Penauts strip where Lucy said to Charlie Brown "the world can't end today because somewhere in the world its always tomorrow". Funny, but makes some sense when you think about it.
Dec 21st begins in the middle of the Pacific....for most of the world it will still be the 20th. Will the world end as soon as the 21st begins? How funny if it did - it means that for most Americans the world will end early morning on the 20th, while most are still in bed. So they'll never see "December 21st" by their clocks.
Or will the world end when the world has spun one more time and the 22nd is about to dawn in the Pacific? Still a shocker for the US, because it'll STILL only be early morning of the 21st. Many still asleep, many still thinking they've got a dozen or more hours before it's "doomsday time". But that's the thing - when the 21st is ending for the Americans (say, at 9pm Central), its already lunchtime on the 22nd for the Japanese, celebrating the fact that the world didn't end on the 21st.
Maybe the world will end in "stripes" around the world as the 21st begins. Worst luck for the Americans again, as they'll have 21 hours of news reports and panics as the world slowly vanishes like segments falling from an orange.
Probably the "most likely" scenario.......
.....is that the world will end at Mayan time.....but is that dawn? Midnight at the start of the 21st? Midnight at the end?
In short, its one heck of a mess, with the only scenario with the world ending on the 21st for EVERYONE being the third paragraph above, with America going "poof" sometime between 3 and 7am in the morning. So basically, if you live in the US, and you think the world will end "on the 21st".....you'd better say all your goodbyes on the 22nd, 'cos its all going to end while you're snoring in your beds.
There's worse ways to go, I suppose.....but writing the above has just reinforced for me how ridiculous it is for anyone to be saying "the world will end on (date)!"