code: @Override
      protected Date handleUnparsableDateString(final String value) throws ConversionException {
  
          try {
              return durationFormat.parse(value);
  
          } catch (final ParseException e1) {
              try {
  
                  return additionalFormat.parse("000000".substring(value.length() <= 6 ? value.length() : 6) + value);
              } catch (final ParseException e2) {
                  // if Parsing is not possible ConversionException is thrown
              }
          }
          throw new ConversionException("input is not in HH:MM:SS format.");
      }
docs: This method is called to handle a non-empty date string from the client
  if the client could not parse it as a Date. In the current case two
  different parsing schemas are tried. If parsing is not possible a
  ConversionException is thrown which marks the DurationField as invalid.