The 3 elements needed to have a nice JavaScriptSerializer DateTime experience in older browsers and using ASP.NET WebForms

DateTimeJavaScriptConverter.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Web.Script.Serialization;

namespace MyNamespace
{
  /// <summary>
  /// A custom DateTime JavaScriptConverter courtesy of these good folks: http://blog.calyptus.eu/seb/2011/12/custom-datetime-json-serialization/
  /// Using this forces DataTimes to be serialised as ISO 8601 rather "\/Date(1249335477787)\/" style
  /// </summary>
  public class DateTimeJavaScriptConverter : JavaScriptConverter
  {
    public override object Deserialize(IDictionary<string, object> dictionary, Type type, JavaScriptSerializer serializer)
    {
      return new JavaScriptSerializer().ConvertToType(dictionary, type);
    }

    public override IDictionary<string, object> Serialize(object obj, JavaScriptSerializer serializer)
    {
      if (!(obj is DateTime)) return null;
      return new CustomString(((DateTime)obj).ToUniversalTime().ToString("O"));
    }

    public override IEnumerable<Type> SupportedTypes
    {
      get { return new[] { typeof(DateTime) }; }
    }

    private class CustomString : Uri, IDictionary<string, object>
    {
      public CustomString(string str)
        : base(str, UriKind.Relative)
      {
      }

      void IDictionary<string, object>.Add(string key, object value) { throw new NotImplementedException(); }
      bool IDictionary<string, object>.ContainsKey(string key) { throw new NotImplementedException(); }
      ICollection<string> IDictionary<string, object>.Keys { get { throw new NotImplementedException(); } }
      bool IDictionary<string, object>.Remove(string key) { throw new NotImplementedException(); }
      bool IDictionary<string, object>.TryGetValue(string key, out object value) { throw new NotImplementedException(); }
      ICollection<object> IDictionary<string, object>.Values { get { throw new NotImplementedException(); } }
      object IDictionary<string, object>.this[string key]
      {
        get { throw new NotImplementedException(); }
        set { throw new NotImplementedException(); }
      }
      void ICollection<KeyValuePair<string, object>>.Add(KeyValuePair<string, object> item) { throw new NotImplementedException(); }
      void ICollection<KeyValuePair<string, object>>.Clear() { throw new NotImplementedException(); }
      bool ICollection<KeyValuePair<string, object>>.Contains(KeyValuePair<string, object> item) { throw new NotImplementedException(); }
      void ICollection<KeyValuePair<string, object>>.CopyTo(KeyValuePair<string, object>[] array, int arrayIndex) { throw new NotImplementedException(); }
      int ICollection<KeyValuePair<string, object>>.Count { get { throw new NotImplementedException(); } }
      bool ICollection<KeyValuePair<string, object>>.IsReadOnly { get { throw new NotImplementedException(); } }
      bool ICollection<KeyValuePair<string, object>>.Remove(KeyValuePair<string, object> item) { throw new NotImplementedException(); }
      IEnumerator<KeyValuePair<string, object>> IEnumerable<KeyValuePair<string, object>>.GetEnumerator() { throw new NotImplementedException(); }
      IEnumerator IEnumerable.GetEnumerator() { throw new NotImplementedException(); }
    }
  }
}

parseISO8601Date.js

//===============================================================================
// Parse ISO 8601 Date Format date string and return a date - equivalent to https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Date/parse
// Found here: n8v.enteuxis.org/2010/12/parsing-iso-8601-dates-in-javascript/
//===============================================================================
function parseISO8601Date(s) {

  // parenthese matches:
  // year month day    hours minutes seconds  
  // dotmilliseconds 
  // tzstring plusminus hours minutes
  var re = /(\d{4})-(\d\d)-(\d\d)T(\d\d):(\d\d):(\d\d)(\.\d+)?(Z|([+-])(\d\d):(\d\d))/;

  var d = [];
  d = s.match(re);

  // "2010-12-07T11:00:00.000-09:00" parses to:
  //  ["2010-12-07T11:00:00.000-09:00", "2010", "12", "07", "11",
  //     "00", "00", ".000", "-09:00", "-", "09", "00"]
  // "2010-12-07T11:00:00.000Z" parses to:
  //  ["2010-12-07T11:00:00.000Z",      "2010", "12", "07", "11", 
  //     "00", "00", ".000", "Z", undefined, undefined, undefined]

  if (!d) {
    throw "Couldn't parse ISO 8601 date string '" + s + "'";
  }

  // parse strings, leading zeros into proper ints
  var a = [1, 2, 3, 4, 5, 6, 10, 11];
  for (var i in a) {
    d[a[i]] = parseInt(d[a[i]], 10);
  }
  d[7] = parseFloat(d[7]);

  // Date.UTC(year, month[, date[, hrs[, min[, sec[, ms]]]]])
  // note that month is 0-11, not 1-12
  // see https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Date/UTC
  var ms = Date.UTC(d[1], d[2] - 1, d[3], d[4], d[5], d[6]);

  // if there are milliseconds, add them
  if (d[7] > 0) {
    ms += Math.round(d[7] * 1000);
  }

  // if there's a timezone, calculate it
  if (d[8] != "Z" && d[10]) {
    var offset = d[10] * 60 * 60 * 1000;
    if (d[11]) {
      offset += d[11] * 60 * 1000;
    }
    if (d[9] == "-") {
      ms -= offset;
    }
    else {
      ms += offset;
    }
  }

  return new Date(ms);
}

web.config

<configuration>
  <system.web.extensions>
    <scripting>
      <webServices>
        <!-- 
          This line of config means that when a JavaScriptSerializer is used by a web service / page method
          it will automatically register the DateTimeJavaScriptConverter to use.  To use the converter directly in code you would need to enter the below:
          
          var serializer = new System.Web.Script.Serialization.JavaScriptSerializer();
          serializer.RegisterConverters(new JavaScriptConverter[] { new DateTimeJavaScriptConverter() });

        -->
        <jsonSerialization>
          <converters>
            <add name="DateTimeJavaScriptConverter" type="MyNamespace.DateTimeJavaScriptConverter"/>
          </converters>
        </jsonSerialization>

      </webServices>
      <scriptResourceHandler enableCompression="false" enableCaching="true"/>
    </scripting>
  </system.web.extensions>
</configuration>