Index: modules/user/user.install
===================================================================
RCS file: /cvs/drupal/drupal/modules/user/user.install,v
retrieving revision 1.5
diff -u -r1.5 user.install
--- modules/user/user.install	8 Jan 2008 07:46:41 -0000	1.5
+++ modules/user/user.install	9 Jan 2008 20:59:11 -0000
@@ -219,7 +219,8 @@
       'timezone' => array(
         'type' => 'varchar',
         'length' => 8,
-        'not null' => FALSE,
+        'not null' => TRUE,
+        'default' => 0,
         'description' => t("User's timezone."),
       ),
       'language' => array(
Index: modules/system/system.install
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.install,v
retrieving revision 1.227
diff -u -r1.227 system.install
--- modules/system/system.install	9 Jan 2008 09:51:35 -0000	1.227
+++ modules/system/system.install	9 Jan 2008 21:11:45 -0000
@@ -2449,6 +2449,16 @@
 }
 
 /**
+ * Apply a default timezone when the user did not edit his account.
+ */
+function system_update_6046() {
+  $ret = array();
+  db_change_field($ret, 'users', 'timezone', 'timezone', array('type' => 'varchar', 'length' => 8, 'not null' => TRUE, 'default' => '0'));
+  $ret[] = update_sql("UPDATE {users} SET timezone = '0' WHERE status = NULL");
+  return $ret;
+}
+
+/**
  * @} End of "defgroup updates-5.x-to-6.x"
  * The next series of updates should start at 7000.
  */