title: insert row with datetime index to dataframe
body: <p>I have a dataframe with datetime index:</p>
  
  <pre><code>time                count  day hour  minute  label
  2018-06-07 00:25:00 207    7.0 0.0   25.0    177.0
  2018-06-07 00:55:00 187    7.0 0.0   55.0    150.0
  2018-06-07 01:25:00 194    7.0 1.0   25.0    165.0
  2018-06-07 01:55:00 176    7.0 1.0   55.0    175.0
  2018-06-07 02:25:00 195    7.0 2.0   25.0    172.0
  -&gt; add new datetime record record here 
  </code></pre>
  
  <p>and I'm trying to add some new records but I get:</p>
  
  <pre><code>[DatetimeIndex(['2018-06-07 01:55:00'], dtype='datetime64[ns]', name='time', freq=None)] not an index
  # this happen even if row exists or not
  </code></pre>
  
  <p>I just want to add a 30 minutes interval record, my code is something like</p>
  
  <pre><code>last_date = recent_posts.iloc[[-1]].index
  last_date = last_date + timedelta(minutes=30)
  
  recent_posts.iloc[[last_date]] = # bla #bla
  
  # What I may be doing wrong?
  </code></pre>