Avoid django DST error: bug 1092647

Use tz-aware datetime in query to avoid automatic conversion by
django.utils.timezone.make_aware, which will throw an error during DST
change, because Zen. https://code.djangoproject.com/ticket/22598
Yes, we should grep for all other uses of `now` or `utcnow` and ensure we're using the one from `django.utils.timezone` as well.