code: public static function DAYOFMONTH($dateValue = 1)
      {
          $dateValue = Functions::flattenSingleValue($dateValue);
  
          if ($dateValue === null) {
              $dateValue = 1;
          } elseif (is_string($dateValue = self::getDateValue($dateValue))) {
              return Functions::VALUE();
          }
  
          if (Functions::getCompatibilityMode() == Functions::COMPATIBILITY_EXCEL) {
              if ($dateValue < 0.0) {
                  return Functions::NAN();
              } elseif ($dateValue < 1.0) {
                  return 0;
              }
          }
  
          // Execute function
          $PHPDateObject = Date::excelToDateTimeObject($dateValue);
  
          return (int) $PHPDateObject->format('j');
      }
docs: DAYOFMONTH.
  
  Returns the day of the month, for a specified date. The day is given as an integer
  ranging from 1 to 31.
  
  Excel Function:
  DAY(dateValue)
  
  @param mixed $dateValue Excel date serial value (float), PHP date timestamp (integer),
  PHP DateTime object, or a standard date string
  
  @return int Day of the month