/**
 * Returns a description of this past date in relative terms.
 * Takes an optional parameter (default: 0) setting the threshold in ms which
 * is considered "Just now".
 *
 * Examples, where new Date().toString() == "Mon Nov 23 2009 17:36:51 GMT-0500 (EST)":
 *
 * new Date().toRelativeTime()
 * --> 'Just now'
 *
 * new Date("Nov 21, 2009").toRelativeTime()
 * --> '2 days ago'
 *
 * // One second ago
 * new Date("Nov 23 2009 17:36:50 GMT-0500 (EST)").toRelativeTime()
 * --> '1 second ago'
 *
 * // One second ago, now setting a now_threshold to 5 seconds
 * new Date("Nov 23 2009 17:36:50 GMT-0500 (EST)").toRelativeTime(5000)
 * --> 'Just now'
 *
 */
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, "ago"].join(" ");
};

/*
 * Wraps up a common pattern used with this plugin whereby you take a String
 * representation of a Date, and want back a date object.
 */
Date.fromString = function(str) {
  return new Date(Date.parse(str));
};

http://github.com/jherdman/javascript-relative-time-helpers/blob/master/date.extensions.js

Relative time extension

JavaScript