title: Datetime in pandas dataframe will not subtract from each other
body: <p>I am trying to find the difference in times between two columns in a pandas dataframe both in datetime format.</p>
  
  <p>Below is some of the data in my dataframe and the code I have been using. I have triple checked that these two columns dtypes are datetime64.</p>
  
  <p>My data:</p>
  
  <pre><code>date_updated                  date_scored 
  2016-03-30 08:00:00.000       2016-03-30 08:00:57.416  
  2016-04-07 23:50:00.000       2016-04-07 23:50:12.036 
  </code></pre>
  
  <p>My code:</p>
  
  <pre><code>data['date_updated'] = pd.to_datetime(data['date_updated'], 
  format='%Y-%m-%d %H:%M:%S')
  data['date_scored'] = pd.to_datetime(data['date_scored'], 
  format='%Y-%m-%d %H:%M:%S')
  data['Diff'] =  data['date_updated'] - data['date_scored']
  </code></pre>
  
  <p>The error message I receive:</p>
  
  <pre><code>TypeError: data type "datetime" not understood
  </code></pre>
  
  <p>Any help would be appreciated, thanks!</p>
  
  <p>My work around solution:</p>
  
  <pre><code>for i in raw_data[:10]:
  scored = i.date_scored
  scored_date =  pd.to_datetime(scored, format='%Y-%m-%d %H:%M:%S')
  if type(scored_date) == "NoneType":
      pass
  elif scored_date.year &gt;= 2016:
      extracted = i.date_extracted
      extracted =  pd.to_datetime(extracted, format='%Y-%m-%d %H:%M:%S')
      bank = i.bank.name
      diff = scored - extracted
      datum = [str(bank), str(extracted), str(scored), str(diff)]
      data.append(datum)
  else:
      pass
  </code></pre>