title: Read column names as date format
body: <p>I have date in Excel as follows, first <code>col1</code> as char and from <code>col2</code> to <code>col5</code> is in date format (mm/dd/yyyy)</p>
  
  <pre><code>id   1/1/2016   2/1/2016  3/1/2016  4/1/2016  5/1/2016
  1     23         545       33         55          66
  2     454        536       66         80          11
  3     83         585        9         10          19
  </code></pre>
  
  <p>I tried to import the above file in to R using <code>readxl</code> library, and my result shows column names which are in date format shows as number in dataset,</p>
  
  <p>How to import Excel date column with same format?</p>