Contact Summary Hook Implementation
As already pointed out, the created date is shown on the summary anyway - but since your question is more about using hook_civicrm_summary(), here's an simple example to add the date above the existing summary content:

function demo_civicrm_summary($contactID, &amp;$content, &amp;$contentPlacement) {
  // Lookup 'created_date' for the specified contact
  $dateCreated = civicrm_api3('Contact', 'getValue', array(
    'id' =&gt; $contactID, 
    'return' =&gt; 'created_date',
  ));  

  // Set $content to what we want to display
  // Can add html with classes for formatting ...
  $content = 'Date created: ' . $dateCreated;

  // Set $contentPlacement to specify where our content is displayed.
  // The default if we don't set this is to display below the existing summary
  $contentPlacement = CRM_Utils_Hook::SUMMARY_ABOVE;
}