title: Can't index by timestamp in pandas dataframe
body: <p>I took an excel sheet which has dates and some values and want to convert them to pandas dataframe and select only rows which are between certain dates.</p>
  
  <p>For some reason I cannot select a row by date index</p>
  
  <p>Raw Data in Excel file</p>
  
  <pre><code>MCU                         
  Timestamp   50D 10P1    10P2    10P3    10P6    10P9    10P12
  12-Feb-15   25.17   5.88    5.92    5.98    6.18    6.23    6.33
  11-Feb-15   25.9    6.05    6.09    6.15    6.28    6.31    6.39
  10-Feb-15   26.38   5.94    6.05    6.15    6.33    6.39    6.46
  </code></pre>
  
  <p>Code</p>
  
  <pre><code>xls = pd.ExcelFile('e:/Data.xlsx')
  vols = xls.parse(asset.upper()+'VOL',header=1)
  vols.set_index('Timestamp',inplace=True)
  </code></pre>
  
  <p>Data before set_index</p>
  
  <pre><code>      Timestamp    50D  10P1  10P2  10P3  10P6  10P9  10P12  25P1  25P2  \
  0    2015-02-12  25.17  5.88  5.92  5.98  6.18  6.23   6.33  2.98  3.08   
  1    2015-02-11  25.90  6.05  6.09  6.15  6.28  6.31   6.39  3.12  3.17   
  2    2015-02-10  26.38  5.94  6.05  6.15  6.33  6.39   6.46  3.01  3.16  
  </code></pre>
  
  <p>Data after set_index</p>
  
  <pre><code>              50D  10P1  10P2  10P3  10P6  10P9  10P12  25P1  25P2  25P3  \
  Timestamp                                                                  
  2015-02-12  25.17  5.88  5.92  5.98  6.18  6.23   6.33  2.98  3.08  3.21   
  2015-02-11  25.90  6.05  6.09  6.15  6.28  6.31   6.39  3.12  3.17  3.32   
  2015-02-10  26.38  5.94  6.05  6.15  6.33  6.39   6.46  3.01  3.16  3.31  
  </code></pre>
  
  <p>Output</p>
  
  <pre><code>&gt;&gt;&gt; vols.index
  &lt;class 'pandas.tseries.index.DatetimeIndex'&gt;
  [2015-02-12, ..., NaT]
  Length: 1478, Freq: None, Timezone: None
  
  &gt;&gt;&gt; vols[date(2015,2,12)]
  *** KeyError: datetime.date(2015, 2, 12)
  </code></pre>
  
  <p>I would expect this not to fail, and also I should be able to select a range of dates. Tried so many combinations but not getting it.</p>