diff --git a/includes/common.inc b/includes/common.inc
index 339a69b..b378528 100644
--- a/includes/common.inc
+++ b/includes/common.inc
@@ -2066,16 +2066,30 @@ function format_date($timestamp, $type = 'medium', $format = '', $timezone = NUL
   // input string.
   // Paired backslashes are isolated to prevent errors in read-ahead evaluation.
   // The read-ahead expression ensures that A matches, but not \A.
+  $original_format = $format;
   $format = preg_replace(array('/\\\\\\\\/', '/(?<!\\\\)([AaeDlMTF])/'), array("\xEF\\\\\\\\\xFF", "\xEF\\\\\$1\$1\xFF"), $format);
 
   // Call date_format().
-  $format = date_format($date_time, $format);
+  $formatted_date = date_format($date_time, $format);
 
   // Pass the langcode to _format_date_callback().
   _format_date_callback(NULL, $langcode);
 
   // Translate the marked sequences.
-  return preg_replace_callback('/\xEF([AaeDlMTF]?)(.*?)\xFF/', '_format_date_callback', $format);
+  $formatted_date = preg_replace_callback('/\xEF([AaeDlMTF]?)(.*?)\xFF/', '_format_date_callback', $formatted_date);
+
+  // Allow modules to alter the result.
+  $context = array(
+    'date_time' => $date_time,
+    'timestamp' => $timestamp,
+    'type' => $type,
+    'format' => $original_format,
+    'timezone' => $timezone,
+    'langcode' => $langcode
+  );
+  drupal_alter('format_date', $formatted_date, $context);
+
+  return filter_xss_admin($formatted_date);
 }
 
 /**
diff --git a/modules/simpletest/tests/system_test.module b/modules/simpletest/tests/system_test.module
index fef539a..07472af 100644
--- a/modules/simpletest/tests/system_test.module
+++ b/modules/simpletest/tests/system_test.module
@@ -569,3 +569,11 @@ function system_test_module_implements_alter(&$implementations, $hook) {
     $implementations = array_merge(array_slice($implementations, 0, $count / 2, TRUE), array('system_test' => $group), array_slice($implementations, $count / 2, NULL, TRUE));
   }
 }
+
+/**
+ * Implements hook_format_date_alter().
+ */
+function system_test_format_date_alter(&$formatted_date, array $context) {
+  // Reverse the date string for testing purposes.
+  $formatted_date = strrev($formatted_date);
+}
diff --git a/modules/system/system.api.php b/modules/system/system.api.php
index 3152139..f570201 100644
--- a/modules/system/system.api.php
+++ b/modules/system/system.api.php
@@ -4802,6 +4802,26 @@ function hook_filetransfer_info_alter(&$filetransfer_info) {
 }
 
 /**
+ * Alter the formatted date.
+ *
+ * @param string $formatted_date
+ *   The formatted date.
+ * @param array $context
+ *   An associative array containing:
+ *   - date_time: The PHP DateTime object with the date to format.
+ *   - timestamp: The Unix timestamp.
+ *   - type: The format to use as passed to format_date().
+ *   - format: If type is 'custom', a PHP date format string suitable for input
+ *     to date().
+ *   - langcode: The language code as passed to format_date().
+ *
+ * @see format_date()
+ */
+function hook_format_date_alter(&$formatted_date, array $context) {
+  $formatted_date .= ' ' . $context['type'];
+}
+
+/**
  * @} End of "addtogroup hooks".
  */
 
diff --git a/modules/system/system.test b/modules/system/system.test
index ec71093..c8fb2d8 100644
--- a/modules/system/system.test
+++ b/modules/system/system.test
@@ -1459,6 +1459,30 @@ class DateTimeFunctionalTest extends DrupalWebTestCase {
       ->fetchColumn();
     $this->assertFalse($format, 'Localized date format for disabled language is ignored.');
   }
+
+  /**
+   * Test format_date_alter.
+   */
+  function testFormatDateAlter() {
+    module_enable(array('system_test'));
+    // Setup date/time settings for Honolulu time.
+    variable_set('date_default_timezone', 'Pacific/Honolulu');
+    variable_set('configurable_timezones', 0);
+    variable_set('date_format_medium', 'Y-m-d H:i:s O');
+
+    // 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.');
+    module_disable(array('system_test'));
+  }
 }
 
 class PageTitleFiltering extends DrupalWebTestCase {