title: Pandas select columns with boolean date condition
body: <p>I'd like to use a boolean index to select columns from a pandas dataframe with a datetime index as the column header:</p>
  
  <pre><code>dates = pd.date_range('20130101', periods=6)
  df = pd.DataFrame(np.random.randn(4, 6), index=list('ABCD'), columns=dates)
  </code></pre>
  
  <p>returns:</p>
  
  <pre><code>   2013-01-01  2013-01-02  2013-01-03  2013-01-04  2013-01-05  2013-01-06
  A    0.173096    0.344348    1.059990   -1.246944    1.624399   -0.276052
  B    0.277148    0.965226   -1.301612   -1.264500   -0.124489    1.704485
  C   -0.375106    0.103812    0.939749   -2.826329   -0.275420    0.664325
  D    0.039756    0.631373    0.643565   -1.516543   -0.654626   -1.544038
  </code></pre>
  
  <p>I'd like to return only the first three columns.</p>