title: python pandas dataframe slicing by date conditions
body: <p>I am able to read and slice pandas dataframe using python datetime objects, however I am forced to use only <em>existing dates</em> in index.  For example, this works:</p>
  
  <pre><code>&gt;&gt;&gt; data
  &lt;class 'pandas.core.frame.DataFrame'&gt;
  DatetimeIndex: 252 entries, 2010-12-31 00:00:00 to 2010-04-01 00:00:00
  Data columns:
  Adj Close    252  non-null values
  dtypes: float64(1)
  
  &gt;&gt;&gt; st = datetime.datetime(2010, 12, 31, 0, 0)
  &gt;&gt;&gt; en = datetime.datetime(2010, 12, 28, 0, 0)
  
  &gt;&gt;&gt; data[st:en]
              Adj Close
  Date                 
  2010-12-31     593.97
  2010-12-30     598.86
  2010-12-29     601.00
  2010-12-28     598.92
  </code></pre>
  
  <p>However if I use a start or end date that is not present in the DF, I get python KeyError.</p>
  
  <p>My Question : How do I query the dataframe object for a date range; even when the start and end dates are not present in the DataFrame.  Does pandas allow for range based slicing?</p>
  
  <p>I am using pandas version 0.10.1</p>