code: private function getMonthAdjuster()
      {
          static $adjuster = null;
          if (!isset($adjuster)) {
              $adjuster = new FieldAdjuster(self::$MONTH, self::$YEAR, 1, 12);
          }
          return $adjuster;
      }
docs: 月の調整をするための FieldAdjuster を返します.
  
  @return FieldAdjuster
  @codeCoverageIgnore