diff --git a/django/test/signals.py b/django/test/signals.py
index ed16831d42..81808dfa3c 100644
--- a/django/test/signals.py
+++ b/django/test/signals.py
@@ -1,27 +1,47 @@
+import os
+import time
+
 from django.conf import settings
 from django.db import connections
 from django.dispatch import receiver, Signal
 from django.template import context
+from django.utils import timezone
 
 template_rendered = Signal(providing_args=["template", "context"])
 
 setting_changed = Signal(providing_args=["setting", "value"])
 
+
 @receiver(setting_changed)
 def update_connections_time_zone(**kwargs):
+    if kwargs['setting'] == 'TIME_ZONE':
+        # Reset process time zone
+        if hasattr(time, 'tzset'):
+            if kwargs['value']:
+                os.environ['TZ'] = kwargs['value']
+            else:
+                os.environ.pop('TZ', None)
+            time.tzset()
+
+        # Reset local time zone cache
+        timezone._localtime = None
+
+    # Reset the database connections' time zone
     if kwargs['setting'] == 'USE_TZ' and settings.TIME_ZONE != 'UTC':
         USE_TZ, TIME_ZONE = kwargs['value'], settings.TIME_ZONE
     elif kwargs['setting'] == 'TIME_ZONE' and not settings.USE_TZ:
         USE_TZ, TIME_ZONE = settings.USE_TZ, kwargs['value']
-    else:   # no need to change the database connnections' time zones
+    else:
+        # no need to change the database connnections' time zones
         return
-
     tz = 'UTC' if USE_TZ else TIME_ZONE
     for conn in connections.all():
+        conn.settings_dict['TIME_ZONE'] = tz
         tz_sql = conn.ops.set_time_zone_sql()
         if tz_sql:
             conn.cursor().execute(tz_sql, [tz])
 
+
 @receiver(setting_changed)
 def clear_context_processors_cache(**kwargs):
     if kwargs['setting'] == 'TEMPLATE_CONTEXT_PROCESSORS':
diff --git a/django/utils/timezone.py b/django/utils/timezone.py
index 0bba97e736..d9d9636023 100644
--- a/django/utils/timezone.py
+++ b/django/utils/timezone.py
@@ -95,7 +95,6 @@ utc = pytz.utc if pytz else UTC()
 
 # In order to avoid accessing the settings at compile time,
 # wrap the expression in a function and cache the result.
-# If you change settings.TIME_ZONE in tests, reset _localtime to None.
 _localtime = None
 
 def get_default_timezone():
diff --git a/tests/modeltests/timezones/tests.py b/tests/modeltests/timezones/tests.py
index 7505503d1f..90bc1f3a18 100644
--- a/tests/modeltests/timezones/tests.py
+++ b/tests/modeltests/timezones/tests.py
@@ -50,32 +50,8 @@ requires_tz_support = skipUnless(TZ_SUPPORT,
         "time zone, but your operating system isn't able to do that.")
 
 
-class BaseDateTimeTests(TestCase):
-
-    @classmethod
-    def setUpClass(self):
-        self._old_time_zone = settings.TIME_ZONE
-        settings.TIME_ZONE = connection.settings_dict['TIME_ZONE'] = 'Africa/Nairobi'
-        timezone._localtime = None
-        if TZ_SUPPORT:
-            self._old_tz = os.environ.get('TZ')
-            os.environ['TZ'] = 'Africa/Nairobi'
-            time.tzset()
-
-    @classmethod
-    def tearDownClass(self):
-        settings.TIME_ZONE = connection.settings_dict['TIME_ZONE'] = self._old_time_zone
-        timezone._localtime = None
-        if TZ_SUPPORT:
-            if self._old_tz is None:
-                del os.environ['TZ']
-            else:
-                os.environ['TZ'] = self._old_tz
-            time.tzset()
-
-
-@override_settings(USE_TZ=False)
-class LegacyDatabaseTests(BaseDateTimeTests):
+@override_settings(TIME_ZONE='Africa/Nairobi', USE_TZ=False)
+class LegacyDatabaseTests(TestCase):
 
     def test_naive_datetime(self):
         dt = datetime.datetime(2011, 9, 1, 13, 20, 30)
@@ -269,8 +245,8 @@ class LegacyDatabaseTests(BaseDateTimeTests):
                 transform=lambda d: d)
 
 
-@override_settings(USE_TZ=True)
-class NewDatabaseTests(BaseDateTimeTests):
+@override_settings(TIME_ZONE='Africa/Nairobi', USE_TZ=True)
+class NewDatabaseTests(TestCase):
 
     @requires_tz_support
     def test_naive_datetime(self):
@@ -486,7 +462,8 @@ class NewDatabaseTests(BaseDateTimeTests):
         self.assertEqual(e.dt, None)
 
 
-class SerializationTests(BaseDateTimeTests):
+@override_settings(TIME_ZONE='Africa/Nairobi')
+class SerializationTests(TestCase):
 
     # Backend-specific notes:
     # - JSON supports only milliseconds, microseconds will be truncated.
@@ -639,8 +616,9 @@ class SerializationTests(BaseDateTimeTests):
             obj = serializers.deserialize('yaml', data).next().object
             self.assertEqual(obj.dt.replace(tzinfo=UTC), dt)
 
-@override_settings(DATETIME_FORMAT='c', USE_L10N=False, USE_TZ=True)
-class TemplateTests(BaseDateTimeTests):
+
+@override_settings(DATETIME_FORMAT='c', TIME_ZONE='Africa/Nairobi', USE_L10N=False, USE_TZ=True)
+class TemplateTests(TestCase):
 
     @requires_tz_support
     def test_localtime_templatetag_and_filters(self):
@@ -723,10 +701,8 @@ class TemplateTests(BaseDateTimeTests):
         tpl = Template("{% load tz %}{{ dt|localtime }}|{{ dt|utc }}")
         ctx = Context({'dt': datetime.datetime(2011, 9, 1, 12, 20, 30)})
 
-        timezone._localtime = None
         with self.settings(TIME_ZONE='Europe/Paris'):
             self.assertEqual(tpl.render(ctx), "2011-09-01T12:20:30+02:00|2011-09-01T10:20:30+00:00")
-        timezone._localtime = None
 
         # Use a pytz timezone as argument
         tpl = Template("{% load tz %}{{ dt|timezone:tz }}")
@@ -864,11 +840,9 @@ class TemplateTests(BaseDateTimeTests):
         tpl = Template("{% load tz %}{{ dt }}")
         ctx = Context({'dt': datetime.datetime(2011, 9, 1, 12, 20, 30, tzinfo=EAT)})
 
-        timezone._localtime = None
         with self.settings(TIME_ZONE=None):
             # the actual value depends on the system time zone of the host
             self.assertTrue(tpl.render(ctx).startswith("2011"))
-        timezone._localtime = None
 
     @requires_tz_support
     def test_now_template_tag_uses_current_time_zone(self):
@@ -879,8 +853,8 @@ class TemplateTests(BaseDateTimeTests):
             self.assertEqual(tpl.render(Context({})), "+0700")
 
 
-@override_settings(DATETIME_FORMAT='c', USE_L10N=False, USE_TZ=False)
-class LegacyFormsTests(BaseDateTimeTests):
+@override_settings(DATETIME_FORMAT='c', TIME_ZONE='Africa/Nairobi', USE_L10N=False, USE_TZ=False)
+class LegacyFormsTests(TestCase):
 
     def test_form(self):
         form = EventForm({'dt': u'2011-09-01 13:20:30'})
@@ -914,8 +888,8 @@ class LegacyFormsTests(BaseDateTimeTests):
         self.assertEqual(e.dt, datetime.datetime(2011, 9, 1, 13, 20, 30))
 
 
-@override_settings(DATETIME_FORMAT='c', USE_L10N=False, USE_TZ=True)
-class NewFormsTests(BaseDateTimeTests):
+@override_settings(DATETIME_FORMAT='c', TIME_ZONE='Africa/Nairobi', USE_L10N=False, USE_TZ=True)
+class NewFormsTests(TestCase):
 
     @requires_tz_support
     def test_form(self):
@@ -960,8 +934,8 @@ class NewFormsTests(BaseDateTimeTests):
         self.assertEqual(e.dt, datetime.datetime(2011, 9, 1, 10, 20, 30, tzinfo=UTC))
 
 
-@override_settings(DATETIME_FORMAT='c', USE_L10N=False, USE_TZ=True)
-class AdminTests(BaseDateTimeTests):
+@override_settings(DATETIME_FORMAT='c', TIME_ZONE='Africa/Nairobi', USE_L10N=False, USE_TZ=True)
+class AdminTests(TestCase):
 
     urls = 'modeltests.timezones.urls'
     fixtures = ['tz_users.xml']
@@ -1012,7 +986,8 @@ class AdminTests(BaseDateTimeTests):
         self.assertContains(response, t.created.astimezone(ICT).isoformat())
 
 
-class UtilitiesTests(BaseDateTimeTests):
+@override_settings(TIME_ZONE='Africa/Nairobi')
+class UtilitiesTests(TestCase):
 
     def test_make_aware(self):
         self.assertEqual(