Generating RFC 3339 or ISO 8601 Timestamps in JavaScript

I've been working on a generator for a custom Atom 1.0 feed. The Atom 1.0 specification (RFC 4287) requires Internet Timestamps (RFC 3339) which are based on ISO 8601 but these are not supported by the JavaScript Date object. Failing to find a drop-in JavaScript snippet which could generate the necessary timestamp format, I created the following function. It generates the Internet timestamp string from either the current time or a JavasScript Date object passed as an argument.

/*
 Internet Timestamp Generator
 Copyright (c) 2009 Sebastiaan Deckers
 License: GNU General Public License version 3 or later
*/
var timestamp: function (date) {
 var pad = function (amount, width) {
  var padding = "";
  while (padding.length < width - 1 && amount < Math.pow(10, width - padding.length - 1))
   padding += "0";
  return padding + amount.toString();
 }
 date = date ? date : new Date();
 var offset = date.getTimezoneOffset();
 return pad(date.getFullYear(), 4)
   + "-" + pad(date.getMonth() + 1, 2)
   + "-" + pad(date.getDate(), 2)
   + "T" + pad(date.getHours(), 2)
   + ":" + pad(date.getMinutes(), 2)
   + ":" + pad(date.getSeconds(), 2)
   + "." + pad(date.getMilliseconds(), 3)
   + (offset > 0 ? "-" : "+")
   + pad(Math.floor(Math.abs(offset) / 60), 2)
   + ":" + pad(Math.abs(offset) % 60, 2);
}

Examples:

timestamp();
// "2009-10-13T13:34:52.000+02:00"

timestamp(new Date(1983, 4, 9, 11, 5, 17));
// "1983-05-09T11:05:17.000+02:00"

Hope this helps anyone.