title: Sum TimeField hours/minutes with Pandas
body: <p>I am trying to use Pandas to sum the time (hours, minutes) of a series.  The data comes from a TimeField</p>
  
  <pre><code>class PhoneRecord ( models.Model ):
      et = models.TimeField ( null=True, blank=True )
  </code></pre>
  
  <p>In python I get the record and convert to a dataframe.</p>
  
  <pre><code>phone = PhoneRecord.objects.all()
  
  df = read_frame ( phone )
  
  df.et = df.et.fillna ( '00:00:00' )    # some records are blank
  
  df [ "time" ] = pd.to_datetime(df.et, format = '%H:%M:%S', errors = 'coerce')
  </code></pre>
  
  <p>this gives me the following output.</p>
  
  <pre><code>0      00:00:35
  1      00:00:29
  2      00:00:00
  3      00:00:00
  4      00:00:37
         ......
  </code></pre>
  
  <p>When I try to sum </p>
  
  <pre><code>df.time.sum ()
  </code></pre>
  
  <p>I get errors like:  unsupported operand type(s) for +: 'datetime.time' and 'datetime.time'</p>
  
  <p>What do I need to do to be able to sum and average the data.</p>
  
  <p>Thank you.</p>