public override void ToKML(XmlTextWriter kml )
{
kml.WriteStartElement("TimeSpan");
if((ID != null) && (ID.Length> 0))
kml.WriteAttributeString("id", ID);
if (Begin.dateTime != DateTime.MinValue)
kml.WriteElementString("begin",Begin.ToKML());
if (End.dateTime != DateTime.MinValue)
kml.WriteElementString("end", End.ToKML());
kml.WriteEndElement();
}