Unable to change date format on node created field
One way to do this is using a preprocess hook in your .theme file. There you can do pretty much anything with that variable.

Here an example:

function [theme_name]_preprocess_node(&amp;$variables) {
  if ($variables['display_submitted']) {

    // Getting the node creation timestamp from the node object.
    $date = $variables['node']-&gt;getCreatedTime();

    // Here you can use drupal's format_date() function, or some custom php date formatting.
    $variables['date'] = format_date($date, 'medium');
  }
}


In an earlier beta, before being able to alter the 'date' variable, I had to  first pass the string through the create() method of the SafeString class like this:

$variables['date'] = \Drupal\Core\Render\SafeString::create('your dangerous new date string')


This might not be necessary anymore, concidering all the recent fuss around the safe string functions.
Edit: I still need to do that in beta15.