I don't know where else to put this I'm afraid; I made a modification to your code to allow it to read dates from XML properties as used in PDCalendar; it is a simple change that adds the ability to read an XPath from the config, e.g.

<add key="itemDateProperty" value="testDate/pdcalendarevent/pdcstart"/>

The XPath is a bit of a cheat; the first element is the property alias, then the rest is the XPath for the XML contained in that property.

        public static string GetPropertyValueAsString(this UmbracoDocument document, string propertyAlias)
        {
            String res = String.Empty;

            // Get alias type = name or name / path
            String path = null;
            Match m = Regex.Match(propertyAlias, @"^([\w\d_]+?)(/.+)$");
            if (m.Success)
            {
                propertyAlias = m.Groups[1].Value;
                path = m.Groups[2].Value;
            }
            //

            umbraco.cms.businesslogic.property.Property prop = document.getProperty(propertyAlias);
            if (prop != null)
            {
                if (path != null)
                {
                    XmlDocument doc = new XmlDocument();
                    XmlNode np = prop.ToXml(doc);

                    if (np != null)
                    {
                        XmlNode nv = np.SelectSingleNode(path);

                        if(nv != null)
                            res = (nv is XmlElement ? nv.InnerText : nv.Value);
                    }
                }
                else if(prop.Value != null)
                    res = prop.Value.ToString();

            }

            return res;
        }