thing.js

function beginCountdown (mins) {
  var secs = mins * 60
  var now = new Date()
  var elapsed
  var countdown = setInterval(function(){
    elapsed = new Date() - now
    if (elapsed/1000 < secs) {
      updateProgress(elapsed/1000, secs)
    } else {
      updateProgress(1, 1)
      clearInterval(countdown)
    }
  }, 1000)
  updateProgress(0, 1)
  return countdown
}