title: DatetimeIndex offset with
body: <p>I have a dataframe, generated with the following code:</p>
  
  <pre><code>time_index = pd.date_range(start=datetime(2013, 1, 1, 3),
                         end=datetime(2013, 1, 2, 2, 59),
                         freq='5T')
  grid_columns = [u'in_brd', u'in_alt', u'out_brd', u'out_alt']  
  grid_proto = pd.DataFrame(index=time_index, columns=grid_columns)
  </code></pre>
  
  <p>I've also added some data to this dataframe.</p>
  
  <p>When I'm trying to navigate through the index with <code>int</code> offsets in basic dataframe I get everthing correct:</p>
  
  <pre><code>In[152]: grid_proto.index[0] + 1
  Out[152]: Timestamp('2013-01-01 03:05:00', tz=None)
  </code></pre>
  
  <p>But if I'm trying to work with some kind of slice, I get an error:</p>
  
  <pre><code>In[153]: z = grid_proto[pd.notnull(x.in_brd)]
  In[154]: z.index[0] + 1
  Traceback (most recent call last):
  
  File "&lt;ipython-input-151-3ce8a4e5e2d6&gt;", line 1, in &lt;module&gt;
  z.index[0] + 1
  
  File "tslib.pyx", line 664, in pandas.tslib._Timestamp.__add__ (pandas\tslib.c:12372)
  
  ValueError: Cannot add integral value to Timestamp without offset.
  </code></pre>
  
  <p>I understand that this is because in first case I work with a link to <code>DatetimeIndex</code> elements instead of scalar. And in second case I get exactly scalar <code>Timestamp</code> value of first index element. Am I right?</p>
  
  <p>How to deal with this offset correctly? (I need to navigate through such slice)</p>