title: Python can't subtract offset-naive and offset-aware datetimes
body: <p>I have the following function:</p>
  
  <pre><code>epoch = datetime(1970, 1, 1)
  
  def epoch_seconds(date):
      """Returns the number of seconds from the epoch to date."""
      print(epoch)
      td = date - epoch
      return td.days * 86400 + td.seconds + (float(td.microseconds) / 1000000)
  </code></pre>
  
  <p>When I take a date directly from my model I get the following error:</p>
  
  <blockquote>
    <p>print(epoch_seconds(self.modified))</p>
  </blockquote>
  
  <p><strong>can't subtract offset-naive and offset-aware datetimes</strong></p>
  
  <p>So I checked the format of both <code>self.modified</code> and <code>epoch</code></p>
  
  <pre><code>self.modified = 2015-08-13 16:29:37.601681+00:00
  epoch = 1970-01-01 00:00:00
  </code></pre>
  
  <p>I think I understand the error and need these to be in the same format however, I have no idea what <code>.601681+00:00</code> is and how to do this. Could some help to explain the what is after the <code>.</code> and how to get these to match?</p>