How do I programmatically save "2019-11-04 07:30:00+01"?
There is no DateTime class in Drupal\Core\Datetime you should use DrupalDateTime class instead try with.

    use Drupal\Core\Datetime\DrupalDateTime;
   //$userTimezone = new DateTimeZone(drupal_get_user_timezone());
    $gmtTimezone = new DateTimeZone('GMT');
    $myDateTime = new DrupalDateTime($item['start_date'], $gmtTimezone);
    $dateTime = $myDateTime-&gt;getPhpDateTime();