title: How do I calculate the time difference in a Django template
body: <p>I have a model with two TimeFields. I want to show the time difference in a template (ie, 27 minutes). I see the filter timesince, but it isn't working; I think it only accepts datetimes but my models have stored times only.</p>
  
  <p>Response to @HAL:</p>
  
  <pre><code>'datetime.time' object has no attribute 'year'
  Exception Location: C:\Python27\lib\site-packages\django\utils\timesince.py in timesince, line 32
  </code></pre>