diff --git a/django/utils/cache.py b/django/utils/cache.py
index 4c7d0bb840..7c7a41bafa 100644
--- a/django/utils/cache.py
+++ b/django/utils/cache.py
@@ -73,12 +73,12 @@ def patch_response_headers(response, cache_timeout=None):
     if cache_timeout is None:
         cache_timeout = settings.CACHE_MIDDLEWARE_SECONDS
     now = datetime.datetime.utcnow()
-    expires = now + datetime.timedelta(0, cache_timeout)
     if not response.has_header('ETag'):
         response['ETag'] = md5.new(response.get_content_as_string('utf8')).hexdigest()
     if not response.has_header('Last-Modified'):
         response['Last-Modified'] = now.strftime('%a, %d %b %Y %H:%M:%S GMT')
     if not response.has_header('Expires'):
+        expires = now + datetime.timedelta(0, cache_timeout)
         response['Expires'] = expires.strftime('%a, %d %b %Y %H:%M:%S GMT')
     patch_cache_control(response, max_age=cache_timeout)