Index: includes/install.core.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/install.core.inc,v
retrieving revision 1.49
diff -u -p -r1.49 install.core.inc
--- includes/install.core.inc	3 Jan 2011 15:48:11 -0000	1.49
+++ includes/install.core.inc	7 Feb 2011 03:39:04 -0000
@@ -1433,8 +1433,6 @@ function install_configure_form($form, &
   }
 
   drupal_add_js(drupal_get_path('module', 'system') . '/system.js');
-  // Add JavaScript time zone detection.
-  drupal_add_js('misc/timezone.js');
   // We add these strings as settings because JavaScript translation does not
   // work on install time.
   drupal_add_js(array('copyFieldValue' => array('edit-site-mail' => array('edit-account-mail'))), 'setting');
@@ -1745,6 +1743,11 @@ function _install_configure_form($form, 
     '#weight' => 5,
     '#attributes' => array('class' => array('timezone-detect')),
   );
+  // Add JavaScript time zone detection, only if a time zone hasn't been
+  // selected already.
+  if (!variable_get('date_default_timezone') && empty($form_state['input']['date_default_timezone'])) {
+    $form['server_settings']['date_default_timezone']['#attached']['js'][] = 'misc/timezone.js';
+  }
 
   $form['server_settings']['clean_url'] = array(
     '#type' => 'hidden',