title: django datetimefield: __str__ give different time from admin display
body: <p>i am trying to setup the <strong>str</strong>() to display a datetimefield in admin site. but it give me a different time in different pages.
  the code is like:   </p>
  
  <pre><code>class Order(models.Model):
      order_date = models.DateTimeField(auto_now_add = True,null=True)
      def __str__(self):
          return self.product.name + \
          " ordered on " + \
          str(self.order_date.strftime("%Y%m%d-%H:%M:%S"));
  </code></pre>
  
  <p>it give a time like 20171027-22:28:40 in the list of My_model. However, "Oct. 27, 2017, 3:28 p.m." is displayed if i click into the entry as the pictures. <br>
  <a href="https://i.stack.imgur.com/xUPBE.gif" rel="nofollow noreferrer">enter image description here</a><br>
  <a href="https://i.stack.imgur.com/jbnxF.gif" rel="nofollow noreferrer">enter image description here</a></p>