title: set a pandas datetime64 pandas dataframe column as a datetimeindex without the time component
body: <p>After importing data from a HDF5 file the index for my stock data has disappeared. </p>
  
  <p>One of the columns in my dataframe "Date" is a Datetime64. How do I convert this date column to a <code>datetimeindex</code> column but without the time parts at the end.</p>
  
  <p>So that slicing the dataframe like this <code>data.ix["2016-01-01":"2016-02-06"]</code> works.</p>