function update_end()
{
  var left;
  if (typeof end_challenge === 'undefined')
    left = 14400;
  else
    left = (end_challenge - new Date())/1000;

  if (left < 0) left = 0;

  var heure = Math.floor(left / 3600);
  var min = Math.floor((left / 60) % 60);
  var sec = Math.floor(left % 60);

  $("#hours").html(( heure < 10 ? "0" : "" ) + heure);
  $("#min").html(( min < 10 ? "0" : "" ) + min);
  $("#sec").html(( sec < 10 ? "0" : "" ) + sec);
}

$(document).ready(function() {
   setInterval( function() {
     update_end();
     }, 1000);
});