title: Convert pandas dataframe column of UTC time string to floats
body: <p>I have a pandas dataframe with a column of strings, with datetimes in UTC format, but need to convert them to floats. I'm having trouble doing this. Here is a view of my column:</p>
  
  <pre><code>df['time'][0:3]
  
  0    2018-04-18T19:00:00.000000000Z
  1    2018-04-18T19:15:00.000000000Z
  2    2018-04-18T19:30:00.000000000Z
  Name: time, dtype: object
  </code></pre>
  
  <p>I've been trying this, but isn't working for me:    </p>
  
  <pre><code>import datetime
  for i in range(1,len(df)):
          df['time'][i] = datetime.datetime.strptime(df['time'][i], '%Y-%m-%dT%H:%M:%S.%f000Z')
  </code></pre>
  
  <p>Here is the error I'm trying to fix:</p>
  
  <pre><code>execfile(filename, namespace)
  
  exec(compile(f.read(), filename, 'exec'), namespace)
  
  unsup.fit(np.reshape(df,(-1,df.shape[1])))
  
  X = _check_X(X, self.n_components)
  
  X = check_array(X, dtype=[np.float64, np.float32])
  
  array = np.array(array, dtype=dtype, order=order, copy=copy)
  
  ValueError: could not convert string to float: '2018-06-29T20:45:00.000000000Z'
  </code></pre>
  
  <p>Many thanks in advance. </p>