title: TypeError only with celery
body: <p>I am using Django. So there is an option <code>CELERY_ALWAYS_EAGER</code>.
  When I switch off celery workers with <code>CELERY_ALWAYS_EAGER = True</code>, everything is ok.</p>
  
  <p>When <code>CELERY_ALWAYS_EAGER = False</code> I get an error:</p>
  
  <pre><code>TypeError: unsupported operand type(s) for -: 'unicode' and 'unicode'
  </code></pre>
  
  <p>That's an error when I try to deduct one timestamp from another. Smth like:
  <code>new_date - old_date</code></p>
  
  <p>What's wrong with celery? How can I avoid it?</p>