title: Writing date format using xlwt
body: <p>I'm writing a date using xlwt like this:</p>
  
  <pre><code>date_format = XFStyle()
  date_format.num_format_str = 'dd/MM/yyyy'
  plan.write(1,4,'01/01/2014', date_format)
  </code></pre>
  
  <p>The worksheet is saving fine, with no errors. But even this cell is formatted as date, excel not recognized it as a date until I manually double click it on the excel. But I have more than 1.000 dates and I can't do that all the time.
  Is there a way to save as real date, with no need to manually update the cell?</p>
  
  <p>Thanks</p>