Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Calculation/DateTime.php
public static function getDateValue($dateValue)
    {
        if (!is_numeric($dateValue)) {
            if ((is_string($dateValue)) &&
                (Functions::getCompatibilityMode() == Functions::COMPATIBILITY_GNUMERIC)) {
                return Functions::VALUE();
            }
            if ((is_object($dateValue)) && ($dateValue instanceof \DateTime)) {
                $dateValue = Date::PHPToExcel($dateValue);
            } else {
                $saveReturnDateType = Functions::getReturnDateType();
                Functions::setReturnDateType(Functions::RETURNDATE_EXCEL);
                $dateValue = self::DATEVALUE($dateValue);
                Functions::setReturnDateType($saveReturnDateType);
            }
        }

        return $dateValue;
    }
public static function getDateValue($dateValue)
    {
        if (!is_numeric($dateValue)) {
            if ((is_string($dateValue)) &&
                (Functions::getCompatibilityMode() == Functions::COMPATIBILITY_GNUMERIC)) {
                return Functions::VALUE();
            }
            if ((is_object($dateValue)) && ($dateValue instanceof \DateTime)) {
                $dateValue = Date::PHPToExcel($dateValue);
            } else {
                $saveReturnDateType = Functions::getReturnDateType();
                Functions::setReturnDateType(Functions::RETURNDATE_EXCEL);
                $dateValue = self::DATEVALUE($dateValue);
                Functions::setReturnDateType($saveReturnDateType);
            }
        }

        return $dateValue;
    }
public static function getDateValue ( $ dateValue ) { if ( ! is_numeric ( $ dateValue ) ) { if ( ( is_string ( $ dateValue ) ) && ( Functions :: getCompatibilityMode ( ) == Functions :: COMPATIBILITY_GNUMERIC ) ) { return Functions :: VALUE ( ) ; } if ( ( is_object ( $ dateValue ) ) && ( $ dateValue instanceof \ DateTime ) ) { $ dateValue = Date :: PHPToExcel ( $ dateValue ) ; } else { $ saveReturnDateType = Functions :: getReturnDateType ( ) ; Functions :: setReturnDateType ( Functions :: RETURNDATE_EXCEL ) ; $ dateValue = self :: DATEVALUE ( $ dateValue ) ; Functions :: setReturnDateType ( $ saveReturnDateType ) ; } } return $ dateValue ; }
getDateValue.

@param string $dateValue

@return mixed Excel date/time serial value, or string if error
getDateValue .