How can I convert a date value in ISO 8601 format to a date object in JavaScript?

You should probably use the datejs that f3lix recommended, however I was bored and threw together a little object that does exactly what you asked for:

September 25, 2012: Cleaned code, allow non-extended format, eg 20120925T164740+0200

December 1, 2011: fixed a bug in the month string. August was missing

var ISODate = {
  convert :
    function (input){
      if (!(typeof input === "string")) throw "ISODate, convert: input must be a string";
      var d = input.match(/^(\d{4})-?(\d{2})-?(\d{2})[T ](\d{2}):?(\d{2}):?(\d{2})(\.\d+)?(Z|(?:([+-])(\d{2}):?(\d{2})))$/i);
      if (!d) throw "ISODate, convert: Illegal format";
      return new Date(
        Date.UTC(
          d[1], d[2]-1, d[3],
          d[4], d[5], d[6], d[7] || 0 % 1 * 1000 | 0
        ) + (
          d[8].toUpperCase() === "Z" ? 0 :
            (d[10]*3600 + d[11]*60) * (d[9] === "-" ? 1000 : -1000)
        )
      );
    },
  format :
    function(date, utc){
      if (typeof date === "string") date = this.convert(date);
      if (!(date instanceof Date)) throw "ISODate, format: t is not a date object";

      var t={'FullYear':0, 'Month':0, 'Date':0, 'Hours':0, 'Minutes':0, 'Seconds':0};
      for (var key in t) {
        if (t.hasOwnProperty(key)) t[key] = date["get" +(utc ? "UTC" :"") + key]()
      }

      return this.month[t.Month]
        + " "
        + this.ordinal(t.Date)
        + ", "
        + t.FullYear
        + " @ "
        + this.clock12(t.Hours,t.Minutes);
      },
  month:
    [
      "January", "February", "March", "April", "May", "June",
      "July", "August", "September", "October", "November", "December"
    ],
  ordinal:
    function(n) {
      return n+(
        [
          "th", "st", "nd", "rd"
        ][
          (( n % 100 / 10) | 0) === 1 ? 0 : n % 10 < 4 ? n % 10 : 0
        ]
      );
  },
  clock12:
    function(h24, m, s){
      h24%=24;
      var h12 = (h24 % 12) || 12;
      return h12 + ":" +
        (m < 10 ? "0" + m : m) +
        (isFinite(s) ? ":" + (s < 10 ? "0" + s : s ) : "") +
        (h24 < 12 ? "AM" : "PM");
      }
};

Example:

//Shows the date in the users timezone:
alert(ISODate.format("2007-09-21T14:15:34.058-07:00"));

//Show the date in UTC (Timezone Z, 00:00)
alert(ISODate.format("2007-09-21T14:15:34.058-07:00",true));

Explanation:

convert takes a string as an input and returns a date object if successful or throws an exception if not. The string must be in one of the following formats:

• YYYY-MM-DDThh:mm:ss.sZ
• YYYY-MM-DDThh:mm:ss.sXaa:bb

Where:

• YYYY is the year as an 4 digit integer
• MM is the month as an 2 digit integer
• DD is the date of month as an 2 digit integer
• T is the character T or space (\x20)
• hh is the hour in 24 hour format, as an 2 digit integer
• mm is the minute as an 2 digit integer
• ss.s is the second, either as an 2 digit integer or as a floating point with 2 digits followed by a period followed by one or more digits.
• Z is the character Z (indicating timezone Z, UTC+00:00)
• X is either a plus (+) or minus (-) sign of the timeoffset to UTC
• aa is the hour of timeoffset to UTC as a 2 digit integer
• bb is the minute of timeoffset to ITC as a 2 digit integer

format takes a string in the above format or a date-object and returns a string formated as:

• M D, Y @ h:mm

Where
– M is the full English name of the month
– D is the date of month with a numerical order suffix (1-2 digits)
– Y is the year (1 or more digits)
– h is the hour in 12 hour format (1-2 digits)
– m is the minute (2 digits)

month is an array with the name of the months

ordinal is a function that takes a number as input and return the number with English ordinal suffix.

clock12 is a function that takes hour, minute and second in 24h format and converts it to a string in the US 12h format. The seconds is optional.