Date.prototype.toRelativeTime = function(now_threshold) {
  var delta = new Date() - this;
 
  now_threshold = parseInt(now_threshold, 10);
 
  if (isNaN(now_threshold)) {
    now_threshold = 0;
  }
 
  if (delta <= now_threshold) {
    return 'Just now';
  }
 
  var units = null;
  var conversions = {
    millisecond: 1, // ms    -> ms
    second: 1000,   // ms    -> sec
    minute: 60,     // sec   -> min
    hour:   60,     // min   -> hour
    day:    24,     // hour  -> day
    month:  30,     // day   -> month (roughly)
    year:   12      // month -> year
  };
 
  for (var key in conversions) {
    if (delta < conversions[key]) {
      break;
    } else {
      units = key; // keeps track of the selected key over the iteration
      delta = delta / conversions[key];
    }
  }
 
  // pluralize a unit when the difference is greater than 1.
  delta = Math.floor(delta);
  if (delta !== 1) { units += "s"; }
  return [delta, units].join(" ");
};