Simple countdown timer in JS

gistfile1.js

/**
 * Don't put your vars in the global scope in production code,
 * much less these tersely-named ones.
*/

var end = new Date('July 1, 2016 12:00:00');

var el;

// Seconds/minute, etc (in miliseconds)
var sm = 60 * 1000,
    sh = 3600 * 1000,
    sd = 86400 * 1000;

// Display numbers padded to a certain width
function pad(n, width, z) {
  z = z || '0';
  n = n + '';
  return n.length >= width ? n : new Array(width - n.length + 1).join(z) + n;
}

// Element references are hard-coded; probably don't do this if you put this in production code
function updateTimer() {
  var now = Date.now();
  var then = end.getTime();
  // Math.max is to prevent negative values, which messes up the math
  var dleft = Math.max(Math.floor((then - now) / sd), 0);
  var hleft = Math.max(Math.floor((then - (dleft * sd) - now) / sh), 0);
  var mleft = Math.max(Math.floor((then - (dleft * sd + hleft * sh) - now) / sm), 0);
  var sleft = Math.max(Math.floor((then - (dleft * sd + hleft * sh + mleft * sm) - now) / 1000), 0);
  document.getElementById('dd').innerHTML = pad(dleft, 2);
  document.getElementById('hh').innerHTML = pad(hleft, 2);
  document.getElementById('mm').innerHTML = pad(mleft, 2);
  document.getElementById('ss').innerHTML = pad(sleft, 2);
}

function timer() {
  updateTimer();
  setInterval(updateTimer, 1000);
}