Tweaks to Zach's humane Date script

I like Zach's humane Date script (based on work by Dean Landolt, John Resig, and henrah), but I wanted to be able to pass in an actual Date object in addition to just an ISO8601 date string. I also wanted it to be able to handle future dates (e.g. "10 minutes from now"). Here's the result:

 
/*
 * Javascript Humane Dates
 * Copyright (c) 2008 Dean Landolt (deanlandolt.com)
 * Re-write by Zach Leatherman (zachleat.com)
 * 
 * Adopted from the John Resig's pretty.js
 * at http://ejohn.org/blog/javascript-pretty-date
 * and henrah's proposed modification 
 * at http://ejohn.org/blog/javascript-pretty-date/#comment-297458
 * 
 * Licensed under the MIT license.
 */

 /**
  * @param {String or Date} date_str either an ISO8601 date string or a Date object.
  *          Note: ISO8601 dates are always formatted using UTC/GMT timezone.
  *          Example: 2009-06-03T20:06:44Z
  */
function humane_date(date_str){
 var time_formats = [
  [1, '1 second'],
  [60, 'seconds', 1],
  [90, '1 Minute'], // 60*1.5
  [3600, 'Minutes', 60], // 60*60, 60
  [5400, '1 Hour'], // 60*60*1.5
  [86400, 'Hours', 3600], // 60*60*24, 60*60
  [129600, '1 Day'], // 60*60*24*1.5
  [604800, 'Days', 86400], // 60*60*24*7, 60*60*24
  [907200, '1 Week'], // 60*60*24*7*1.5
  [2628000, 'Weeks', 604800], // 60*60*24*(365/12), 60*60*24*7
  [3942000, '1 Month'], // 60*60*24*(365/12)*1.5
  [31536000, 'Months', 2628000], // 60*60*24*365, 60*60*24*(365/12)
  [47304000, '1 Year'], // 60*60*24*365*1.5
  [3153600000, 'Years', 31536000], // 60*60*24*365*100, 60*60*24*365
  [4730400000, '1 Century'], // 60*60*24*365*100*1.5
 ];

 var time = ('' + date_str).replace(/-/g,"/").replace(/[TZ]/g," "),
  dt = new Date,
  seconds = (date_str instanceof Date ? (dt - date_str)
              : (dt - new Date(time) + (dt.getTimezoneOffset() * 60000))) / 1000,
  token = ' ago',
  i = 0,
  format;

 if (seconds < 0) {
  seconds = Math.abs(seconds);
  token = ' from now';
 }

 while (format = time_formats[i++]) {
  if (seconds < format[0]) {
   if (format.length == 2) {
    return format[1] + token
   } else {
    return Math.round(seconds / format[2]) + ' ' + format[1] + token;
   }
  }
 }

 // overflow for centuries
 if(seconds > 4730400000)
  return Math.round(seconds / 4730400000) + ' Centuries' + token;

 return date_str;
};

if(typeof jQuery != 'undefined') {
 jQuery.fn.humane_dates = function(){
  return this.each(function(){
   var date = humane_date(this.title);
   if(date && jQuery(this).text() != date) // don't modify the dom if we don't have to
    jQuery(this).text(date);
  });
 };
}