From 2007.igem.org

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today = new Date();
jamboree = new Date(2007,11,3);
count = Math.ceil(( jamboree.getTime() - today.getTime() ) / (1000*60*60*24));

if(count > 0) {
document.write('Jamboreeまでのこり', countdown ,'日！');
}
else {
document.write('当日！');
}