Create Custom Date Formatter for Date Posted Field
In Drupal 7, the created and updated "fields" on nodes (and other entities) are properties and not proper fields.  The display values for these are created in the default template_preprocess_node() provided by core, which some theme and modules may override.

Note these lines:

$variables['date'] = format_date($node-&gt;created);
...
// Display post information only on certain node types.
if (variable_get('node_submitted_' . $node-&gt;type, TRUE)) {
  $variables['display_submitted'] = TRUE;
  $variables['submitted'] = t('Submitted by !username on !datetime', array(
    '!username' =&gt; $variables['name'],
    '!datetime' =&gt; $variables['date'],
  ));
  ...
}


So, if you want to change how this is displayed, you need to implement your own template_preprocess_node() (you can do this in a theme or module), and provide your own logic.  You can either update the existing variables, which will allow the existing templates to be used, or you can add new variables and also provide your own node templates.  Typically, I do the later.