today=new Date();                                     //Set the two dates
var christmas=new Date(today.getFullYear(), 11, 25);  //Month is 0-11 in JavaScript
if (today.getMonth()==11 && today.getDate()>25){      //if Christmas has passed already
  christmas.setFullYear(christmas.getFullYear()+1)    //calculate next year's Christmas
}
var one_day=1000*60*60*24                             //Set 1 day in milliseconds
                                                      //Calculate difference btw the two dates, and convert to days
document.write(Math.ceil((christmas.getTime()-today.getTime())/(one_day))+" days left until Christmas!");