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(" ");
};

> now.toRelativeTime()
'30 seconds'
> now.toRelativeTime()
'58 seconds'
> now.toRelativeTime()
'1 minute'