digest tests: Improve gather_new_streams test.

We don't need to mock the dates here.  We also
explicitly clear out all streams first, and then
we explicitly test with both the stream being
current and the stream being old.

zerver/tests/test_digest.py

@@ -25,6 +25,7 @@ from zerver.models import (
     Message,
     Realm,
     RealmAuditLog,
+    Stream,
     UserActivity,
     UserProfile,
     flush_per_request_caches,
@@ -400,16 +401,27 @@ class TestDigestEmailMessages(ZulipTestCase):
             user = arg[0][0]
             self.assertNotEqual(user.id, bot.id)
 
-    @mock.patch('zerver.lib.digest.timezone_now')
     @override_settings(SEND_DIGEST_EMAILS=True)
-    def test_new_stream_link(self, mock_django_timezone: mock.MagicMock) -> None:
-        cutoff = datetime.datetime(year=2017, month=11, day=1, tzinfo=datetime.timezone.utc)
-        mock_django_timezone.return_value = datetime.datetime(year=2017, month=11, day=5, tzinfo=datetime.timezone.utc)
+    def test_new_stream_link(self) -> None:
+        Stream.objects.all().delete()
+        cutoff = timezone_now() - datetime.timedelta(days=5)
         cordelia = self.example_user('cordelia')
-        stream_id = create_stream_if_needed(cordelia.realm, 'New stream')[0].id
-        new_stream = gather_new_streams(cordelia, cutoff)[1]
-        expected_html = f"<a href='http://zulip.testserver/#narrow/stream/{stream_id}-New-stream'>New stream</a>"
-        self.assertIn(expected_html, new_stream['html'])
+        stream = create_stream_if_needed(cordelia.realm, 'New stream')[0]
+        stream.date_created = timezone_now()
+        stream.save()
+
+        stream_count, stream_info = gather_new_streams(cordelia, cutoff)
+        self.assertEqual(stream_count, 1)
+        expected_html = f"<a href='http://zulip.testserver/#narrow/stream/{stream.id}-New-stream'>New stream</a>"
+        self.assertEqual(stream_info['html'][0], expected_html)
+
+        # Make the stream appear to be older.
+        stream.date_created = timezone_now() - datetime.timedelta(days=7)
+        stream.save()
+
+        stream_count, stream_info = gather_new_streams(cordelia, cutoff)
+        self.assertEqual(stream_count, 0)
+        self.assertEqual(stream_info['html'], [])
 
     def simulate_stream_conversation(self, stream: str, senders: List[str]) -> List[int]:
         client = 'website'  # this makes `sent_by_human` return True