public virtual void Write(
     System.Xml.XmlWriter xmlWriter,
     TimeSpan duration,
     string elementName,
     string elementNamespace
     )
 {
     if (duration == null)
         throw new ArgumentNullException("duration");
     this._WriteStartElement(xmlWriter, elementName, elementNamespace);
     this._WriteAttribute
         (
         xmlWriter,
         "interval",
         String.Empty,
         duration.ToISO8601(),
         String.Empty
         );
     this._WriteEndElement(xmlWriter);
 }