code: public static function YEAR($dateValue = 1)
      {
          $dateValue = Functions::flattenSingleValue($dateValue);
  
          if ($dateValue === null) {
              $dateValue = 1;
          } elseif (is_string($dateValue = self::getDateValue($dateValue))) {
              return Functions::VALUE();
          } elseif ($dateValue < 0.0) {
              return Functions::NAN();
          }
  
          // Execute function
          $PHPDateObject = Date::excelToDateTimeObject($dateValue);
  
          return (int) $PHPDateObject->format('Y');
      }
docs: YEAR.
  
  Returns the year corresponding to a date.
  The year is returned as an integer in the range 1900-9999.
  
  Excel Function:
  YEAR(dateValue)
  
  @param mixed $dateValue Excel date serial value (float), PHP date timestamp (integer),
  PHP DateTime object, or a standard date string
  
  @return int Year