Change timestamp format (Non working code)

<?php
// Get Joomla! framework
define('_JEXEC', 1);
define('_VALID_MOS', 1);
define('JPATH_BASE', realpath(dirname(__FILE__)));
define('DS', DIRECTORY_SEPARATOR);

require_once(JPATH_BASE . DS . 'includes' . DS . 'defines.php');
require_once(JPATH_BASE . DS . 'includes' . DS . 'framework.php');

jimport('joomla.database.table');

function parse_rss($rss_link, $additional_title) {
    try {
        $data = array();

        $db = JFactory::getDBO();
        $db->set_charset("utf8");

        $table = JTable::getInstance("content");

        $rawFeed = file_get_contents($rss_link);
        $x = new SimpleXmlElement($rawFeed);

        if (count($x) == 0) {
            return;
        }

        foreach($x->channel->item as $item) {
            try {
                $plink = parse_url($item->link);

                $query = $db->getQuery(true);
                $query->select("id")
                      ->from("jml_content")
                      ->where("introtext LIKE '%" . (string) $plink['path'] . "%'");
                $db->setQuery($query);
                $article_id = $db->loadResult();

                if ($article_id === '' || $article_id == null) {
                    $article_id = $db->insertid();
                }

                $article_title = $item->title;
                if ($additional_title) {
                    $article_title = "xxx $article_title";
                }

                $alias = ltrim((string) $plink['path'], '/');
                $alias = str_replace('/', '-', $alias);

                $data = array();
                $data['id'] = $article_id;
                $data['catid'] = 14;
                $data['title'] = "(" . strftime("%d-%m-%Y", strtotime($item->pubDate)) . ") " . $article_title;
                $data['alias'] = $alias;
                $data['introtext'] = "xxx";
                $data['state'] = 1;
                $data['created'] = strftime("%Y-%m-%d %H:%M:%S", strtotime($item->pubDate));
                $data['publish_up'] = strftime("%Y-%m-%d %H:%M:%S", strtotime($item->pubDate));
                $data['featured'] = 0;
                $data['language'] = "el-GR";

                if (!$table->save($data)) {
                    error_log(print_r($data, true));
                    error_log('Cannot save article. Reason: ');
                    error_log($table->getError());
                }
            }
            catch (Exception $e) {
                error_log('Cannot parse RSS item.');
                error_log("$e->getMessage()");
            }
        }
    }
    catch (Exception $e) {
        error_log('Exception while parsing RSS.\n');
        error_log("$e->getMessage()\n");
    }
}

ob_start();
try {
    date_default_timezone_set( 'Europe/Athens' );
    parse_rss("xxx", false);
    parse_rss("xxx", true);
}
catch (Exception $e) {
    error_log('Cannot parse RSS.\n');
    error_log("$e->getMessage()\n");
}
ob_end_flush();