django fixtures DateTimeField runtimeWarning

I've set some basic fixtures for my django project. One of the records inserted into the database looks like the following:

  {
    "model": "articles.article",
    "pk": 1,
    "fields": {
      "title": "Blackened Recordings Launches",
      "headline": "we're so psyched about our new adventure",
      "content": "<p>We like to make it a point here not to bore you with the not-so-exciting business aspects of making and sharing music, but we're so psyched about our new adventure that we just had to tell you about it as we officially launch our very own record label, Blackened Recordings.</p><p>Some of you, who have followed along throughout the years, are aware that in 1994 we renegotiated our contract with the Warner Music Group, which resulted in a joint venture with our record company for releasing all of our recordings including long form videos. Per that agreement, as of today we have taken ownership of all of our master recordings and Blackened Recordings will be the home of all of our current albums and videos along with all future releases including the December 10 release of the \"Quebec Magnetic\" DVD and Blu-ray.</p><p>You may have heard us say it once or twice or a thousand times before, but it's always been about us taking control of all things 'Tallica to give you 110% on every single level every single time. Forming Blackened Recordings is the ultimate in independence, putting us in the driver's seat of our own creative destiny. We're looking forward to making more music and getting it all out to you in our own unique way.</p>",
      "image": "examples/slide-03.jpg",
      "active": 1,
      "created_at": "2013-03-16 17:41:28"
    }
  },

This is the model it corresponds to:

class Article(models.Model):
    """News article, displayed on homepage to attract users"""
    class Meta:
        db_table = 'article'
    title = models.CharField(max_length=64)
    headline = models.CharField(max_length=255)
    content = models.TextField()
    image = models.ImageField(upload_to = 'articles/', null=True, blank=True)
    active = models.BooleanField()
    created_at = models.DateTimeField()
    def __unicode__(self):
        return self.title

When inserting fixture records, I get the following warning:

/usr/local/lib/python2.7/dist-packages/django/db/models/fields/__init__.py:827: RuntimeWarning: DateTimeField received a naive datetime (2013-03-16 17:41:28) while time zone support is active.
  RuntimeWarning)

I've got no idea what is wrong here. I tried to follow this blog post, but I do have pytz installed and I do have the USE_TZ=True option in my settings.py.

Actually, the solution is hidden deeply in Django docs, quote below:

When serializing an aware datetime, the UTC offset is included, like this:

"2011-09-01T13:20:30+03:00"

Such fixtures are fully accepted, in my case it was:

"2013-03-16T17:41:28+00:00"
"2013-03-17T23:36:12+00:00"
"2013-03-18T13:19:37+00:00"

and the output was:

$ ./manage.py loaddata articles/fixtures/initial_data.json 
Installed 3 object(s) from 1 fixture(s)

Note, that '2013-03-16 17:41:28 UTC+0000' is not proper timezone aware datetime format and it will give you following error:

DeserializationError: Problem installing fixture 'articles/fixtures/initial_data.json': [u"'2013-03-16 17:41:28 UTC+0000' value has an invalid format. It must be in YYYY-MM-DD HH:MM[:ss[.uuuuuu]][TZ] format."]