title: AttributeError: 'str' object has no attribute 'to_datetime'
body: <p>I have a code that reads an excel data sheet (a table) into a <code>DataFrame</code> and convert a 'date' column (with values e.g. 20150508) into date time,</p>
  
  <pre><code>df['date'] = df['date'].astype(str)
  dates = df['date'].to_datetime() // error occurs
  </code></pre>
  
  <p>I got a error,</p>
  
  <pre><code>AttributeError: 'str' object has no attribute 'to_datetime'
  </code></pre>
  
  <p>if I removed the line,</p>
  
  <pre><code>df['date'] = df['date'].astype(str)
  </code></pre>
  
  <p>the other line can run fine, I am wondering what is the problem.</p>