Error importing middleware horizon.middleware: "cannot import name timezone"

when I installed the horizon , From the browser on :
apathe2's error.log:

mod_wsgi (pid=23723): Exception occurred processing WSGI script '/opt/stack/horizon-2012.2.1/openstack_dashboard/wsgi/django.wsgi'.
[Fri Nov 30 06:14:21 2012] [error] [client 192.168.xx.xx] Traceback (most recent call last):
[Fri Nov 30 06:14:21 2012] [error] [client 192.168.xx.xx] File "/usr/lib/pymodules/python2.7/django/core/handlers/wsgi.py", line 250, in __call__
[Fri Nov 30 06:14:21 2012] [error] [client 192.168.xx.xx] self.load_middleware()
[Fri Nov 30 06:14:21 2012] [error] [client 192.168.xx.xx] File "/usr/lib/pymodules/python2.7/django/core/handlers/base.py", line 47, in load_middleware
[Fri Nov 30 06:14:21 2012] [error] [client 192.168.xx.xx] raise exceptions.ImproperlyConfigured('Error importing middleware %s: "%s"' % (mw_module, e))
[Fri Nov 30 06:14:21 2012] [error] [client 192.168.xx.xx] ImproperlyConfigured: Error importing middleware horizon.middleware: "cannot import name timezone"