diff -r -u drupal-8.x-new/core/includes/common.inc drupal-8.x/core/includes/common.inc
--- drupal-8.x-new/core/includes/common.inc	Thu Mar  7 16:43:44 2013
+++ drupal-8.x/core/includes/common.inc	Thu Mar  7 18:17:36 2013
@@ -1899,7 +1899,18 @@
     'langcode' => $langcode,
     'format_string_type' => $key,
   );
-  return filter_xss_admin($date->format($format, $settings));
+  
+  $formatted_date = $date->format($format, $settings);
+  
+  $context = array(
+    'date_time' => $date,
+    'type' => $type,
+    'format' => $format,
+    'langcode' => $langcode,
+  );
+  drupal_alter('format_date', $formatted_date, $context);
+ 
+  return filter_xss_admin($formatted_date);
 }
 
 /**
diff -r -u drupal-8.x-new/core/modules/system/lib/Drupal/system/Tests/Datetime/DrupalDateTimeTest.php drupal-8.x/core/modules/system/lib/Drupal/system/Tests/Datetime/DrupalDateTimeTest.php
--- drupal-8.x-new/core/modules/system/lib/Drupal/system/Tests/Datetime/DrupalDateTimeTest.php	Thu Mar  7 16:43:44 2013
+++ drupal-8.x/core/modules/system/lib/Drupal/system/Tests/Datetime/DrupalDateTimeTest.php	Thu Mar  7 18:13:34 2013
@@ -26,7 +26,7 @@
   /**
    * Set up required modules.
    */
-  public static $modules = array();
+  public static $modules = array('system_test');
 
   /**
    * Test setup.
@@ -108,5 +108,30 @@
     drupal_save_session(TRUE);
 
 
+  }
+
+  /**
+   * Test hook_format_date_alter() function and format_date() function.
+   */
+  function testFormatDate() {
+    // Setup date/time settings for Honolulu time.
+    $config = config('system.timezone')
+      ->set('default', 'Pacific/Honolulu')
+      ->set('user.configurable', 0)
+      ->save();
+    $config = config('system.date')
+      ->set('formats.medium.pattern.php', 'Y-m-d H:i:s O')
+      ->save();
+    // Create some nodes with different authored-on dates.
+    $date1 = '2007-01-31 21:00:00 -1000';
+    $date2 = '2007-07-31 21:00:00 -1000';
+    $node1 = $this->drupalCreateNode(array('created' => strtotime($date1), 'type' => 'article'));
+    $node2 = $this->drupalCreateNode(array('created' => strtotime($date2), 'type' => 'article'));
+
+    // Confirm date format was changed by implemented hook function.
+    $this->drupalGet("node/$node1->nid");
+    $this->assertText('0001- 00:00:12 13-10-7002', 'Date was reversed successfully.');
+    $this->drupalGet("node/$node2->nid");
+    $this->assertText('0001- 00:00:12 13-70-7002', 'Date was reversed successfully.');
   }
 }
diff -r -u drupal-8.x-new/core/modules/system/system.api.php drupal-8.x/core/modules/system/system.api.php
--- drupal-8.x-new/core/modules/system/system.api.php	Thu Mar  7 16:43:44 2013
+++ drupal-8.x/core/modules/system/system.api.php	Thu Mar  7 18:13:34 2013
@@ -3652,6 +3652,29 @@
 }
 
 /**
+ * Alter the formatted date.
+ *
+ * A module may implement this hook in order to alter the formatted date string.
+ *
+ * @param string $formatted_date
+ *   The formatted date.
+ * @param array $context
+ *   An associative array containing:
+ *   - date_time: The DrupalDateTime object with the date to format.
+ *   - type: The format to use as passed to format_date().
+ *   - format: If type is 'custom', a PHP date format string suitable for input
+ *     to date().
+ *   - langcode: The language code as passed to format_date().
+ *
+ * @see format_date()
+ */
+function hook_format_date_alter(&$formatted_date, array $context) {
+  $formatted_date .= ' ' . $context['type'];
+}
+
+
+
+/**
  * @} End of "addtogroup hooks".
  */
 
Only in drupal-8.x/core/modules/system: system.api.php.orig
diff -r -u drupal-8.x-new/core/modules/system/tests/modules/system_test/system_test.module drupal-8.x/core/modules/system/tests/modules/system_test/system_test.module
--- drupal-8.x-new/core/modules/system/tests/modules/system_test/system_test.module	Thu Mar  7 16:43:44 2013
+++ drupal-8.x/core/modules/system/tests/modules/system_test/system_test.module	Thu Mar  7 18:13:34 2013
@@ -380,3 +380,11 @@
   system_authorized_init('system_test_authorize_run', drupal_get_path('module', 'system_test') . '/system_test.module', array(), $page_title);
   drupal_goto($authorize_url);
 }
+
+/**
+ * Implements hook_format_date_alter().
+ */
+function system_test_format_date_alter(&$formatted_date, array $context) {
+  // Reverse the date string for testing purpose.
+  $formatted_date = strrev($formatted_date);
+}
Only in drupal-8.x/core/modules/system/tests/modules/system_test: system_test.module.orig