public Builder addStringDerivedColumn(String columnName,String format,DateTimeZone timeZone){
  derivedColumns.add(new DerivedColumn(columnName,ColumnType.String,format,timeZone,null));
  return this;
}

public Builder addStringDerivedColumn ( String columnName , String format , DateTimeZone timeZone ) { derivedColumns . add ( new DerivedColumn ( columnName , ColumnType . String , format , timeZone , null ) ) ; return this ; }
add a string column ( for example , human readable format ) , derived from the time
