var timer = "";

function start_counting() {
  timer = setInterval('update_count()', 1000)
}

function update_count() {
  day = document.getElementById('days').innerHTML
  hr  = document.getElementById('hours').innerHTML
  min = document.getElementById('minutes').innerHTML
  sec = document.getElementById('seconds').innerHTML
  if(day < 0 || day == 0 &&
      (hr < 0 || hr == 0 && (min < 0 || min == 0 && sec < 0))) {
    if(--sec < -59) {
      sec = 0
      if(--min < -59) {
        min = 0
        if(--hr < -23) {
          hr = 0
          --day
          document.getElementById('days').innerHTML = day
        }
        document.getElementById('hours').innerHTML = hr
      }
      document.getElementById('minutes').innerHTML = min
    }
  } else {
    if(--sec < 0) {
      sec = 59
      if(--min < 0) {
        min = 59
        if(--hr < 0) {
          hr = 23
          --day
          document.getElementById('days').innerHTML = day
        }
        document.getElementById('hours').innerHTML = hr
      }
      document.getElementById('minutes').innerHTML = min
    }
  }
  document.getElementById('seconds').innerHTML = sec
}