title: Pandas DataFrame.loc returns Empty DataFrame
body: <p>Currently trying to work through some tutorials. Right now I have a pandas Dataframe with stock data. The stock data was taken by reading a csv file, then using <code>df.set_index('timestamp', inplace = True)</code> I set the index to use 'timestamp'. The head is seen here:</p>
  
  <pre><code> timestamp   open   high    low   close  adjusted_close   volume  dividend_amount  split_coefficient
  2018-09-11  74.95  75.69  74.76  75.64           75.64  2225700              0.0                1.0
  2018-09-10  75.10  75.21  74.84  74.91           74.91  1774400              0.0                1.0
  2018-09-07  75.20  75.20  74.72  75.01           75.01  1804800              0.0                1.0
  2018-09-06  74.95  75.55  74.62  75.24           75.24  3058300              0.0                1.0
  2018-09-05  75.03  75.42  74.80  74.95           74.95  2418400              0.0                1.0
  </code></pre>
  
  <p>When I try <code>df.loc['2018-09-05']</code>, it returns the correct row to me. However, when I try to select a range, such as <code>df.loc['2018-09-05':'2018-09-11']</code>, I get an Empty DataFrame returned as so:</p>
  
  <pre><code>Empty DataFrame
  Columns: [open, high, low, close, adjusted_close, volume, dividend_amount, 
  split_coefficient]
  Index: []
  </code></pre>
  
  <p>Was wondering if anyone could provide any insight as to why this is happening? I would have expected a return of the information between the two dates instead of the empty dataframe. Thanks!</p>