title: Modify timestamp subset column in pandas
body: <p>I have a Dataframe with a timestamp column:</p>
  
  <pre><code>tij_pd.datetime[0:5]
  Out[29]: 
  0   2016-01-09 05:27:00
  1   2016-01-09 06:49:00
  2   2016-01-09 08:05:00
  3   2016-01-09 12:09:00
  4   2016-01-09 14:54:00
  Name: datetime, dtype: datetime64[ns]
  </code></pre>
  
  <p>I need to select times between '00:00' and '04:00' and add 1Hour.</p>
  
  <pre><code>In[31]:   tij_pd.set_index('datetime').between_time('00:00','04:00').reset_index().datetime
  Out[31]: 
  0    2016-03-09 01:01:00
  1    2016-10-09 00:31:00
  ...
  16   2016-03-09 01:40:00
  17   2016-09-23 00:46:00
  Name: datetime, dtype: datetime64[ns]
  </code></pre>
  
  <p>How can I add 1Hour to the datetime column of this subset?</p>
  
  <pre><code>tij_pd['datetime'] = tij_pd['datetime']+pd.to_timedelta(1,'d')
  </code></pre>