title: Xlwings: date argument conversion
body: <p>In an Excel cell I call function fff(DATE(2001,1,1)). To receive the date argument, I use (xlwings 0.10.0) the following code:</p>
  
  <pre><code>@xw.func
  @xw.arg('req_date', dates=datetime.date) # I also tried datetime.datetime
  def fff(req_date):
      print req_date
  </code></pre>
  
  <p>which prints just a number, not datetime object. I solved the problem by calling</p>
  
  <pre><code>req_date = datetime.datetime(1899, 12, 30)
             + datetime.timedelta(days=req_date)
  </code></pre>
  
  <p>but I wonder what did I do wrong with the xlwings way?! </p>