diff -u b/core/includes/install.core.inc b/core/includes/install.core.inc
--- b/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -1845,18 +1845,18 @@
   );
   $form['update_notifications']['update_status_module'] = array(
     '#type' => 'checkboxes',
-    '#default_value' => array('check', 'notify'),
     '#options' => array(
-      'check' => st('Check for updates automatically.'),
-      'notify' => st('Receive e-mail notifications.'),
+      1 => st('Check for updates automatically'),
+      2 => st('Receive e-mail notifications'),
     ),
+    '#default_value' => array(1, 2),
     '#description' => st('The system will notify you when updates and important security releases are available for installed components. Anonymous information about your site is sent to <a href="@drupal">Drupal.org</a>.', array('@drupal' => 'http://drupal.org')),
     '#weight' => 15,
   );
-  $form['update_notifications']['update_status_module']['notify'] = array(
+  $form['update_notifications']['update_status_module'][2] = array(
     '#states' => array(
       'visible' => array(
-        'input[name="update_status_module[check]"]' => array('checked' => TRUE),
+        'input[name="update_status_module[1]"]' => array('checked' => TRUE),
       ),
     ),
   );
diff -u b/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
--- b/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -2894,16 +2894,16 @@
 }
 
 /**
- * Return the date for a given format string via Ajax.
+ * Ajax callback; Returns the date for a given format string.
  */
-function system_date_time_lookup($form, $form_state) {
+function system_date_time_lookup($form, &$form_state) {
   $format = '';
   if (!empty($form_state['values']['date_format'])) {
     $format = t('Displayed as %date_format', array('%date_format' => format_date(REQUEST_TIME, 'custom', $form_state['values']['date_format'])));
   }
-  // We are returning command and not directly string because if we return
-  // string ajax api will prepend additional empty div element which will mess
-  // our layout.
+  // Return a command instead of a string, since the Ajax framework
+  // automatically prepends an additional empty DIV element for a string, which
+  // breaks the layout.
   $commands[] = ajax_command_replace('#edit-date-format-suffix', '<small id="edit-date-format-suffix">' . $format . '</small>');
   return array('#type' => 'ajax', '#commands' => $commands);
 }
diff -u b/core/modules/system/system.js b/core/modules/system/system.js
--- b/core/modules/system/system.js
+++ b/core/modules/system/system.js
@@ -2,7 +2,7 @@
 
 "use strict";
 
-// Cache ids in an array for ease of use.
+// Cache IDs in an array for ease of use.
 var ids = [];
 
 /**
@@ -39,7 +39,7 @@
 Drupal.behaviors.copyFieldValue = {
   attach: function (context, settings) {
     // List of fields IDs on which to bind the event listener.
-    // Create an array of ids to use with jQuery.
+    // Create an array of IDs to use with jQuery.
     for (var sourceId in settings.copyFieldValue) {
       if (settings.copyFieldValue.hasOwnProperty(sourceId)) {
         ids.push(sourceId);