code: private String concatDate(){
  
          if(params.length != 2){
              throw new IllegalArgumentException("Wrong number of Parameters for concat date expression");
          }
  
          final DateTimeFormat yearFormat = DateTimeFormat.getFormat(DataType.DATE_FORMAT_YEAR_ONLY);
  
          final String rawDateFrom = record.getValueByFieldName(params[0]);
          Date dateFrom = DataType.ParseUtil.tryParseDate(rawDateFrom, DataType.DEFAULT_DATE);
          final String rawDateTo = record.getValueByFieldName(params[1]);
          Date dateTo = DataType.ParseUtil.tryParseDate(rawDateTo, DataType.DEFAULT_DATE);
  
  
          String res;
          if(dateFrom.equals(dateTo)){
              res = yearFormat.format(dateFrom);
          }else{
              res = yearFormat.format(dateFrom) + " - " + yearFormat.format(dateTo);
          }
          return res;
      }
docs: Given 2 fields, concat two years eg 2006 - 2008
  In the config file use:
  fieldName="func(CONCAT_DATE(StudyFrom,StudyTo))"