title: Pandas: convert column of dataframe to datetime
body: <p>I have df                                          </p>
  
  <pre><code>ID month
  0       0001ee12f919a1b570658024bb59d118    2014-02 
  1       0001ee12f919a1b570658024bb59d118    2014-03  
  2       0001ee12f919a1b570658024bb59d118    2014-04  
  3       0001ee12f919a1b570658024bb59d118    2014-05  
  4       0001ee12f919a1b570658024bb59d118    2014-06  
  5       0001ee12f919a1b570658024bb59d118    2014-07
  </code></pre>
  
  <p>and I try to turn <code>year_month</code> to datetime.
  I use <code>df1['month'] = pd.to_datetime(df1.month)</code> 
  but it return <code>ValueError: Unknown string format</code>
  how can I fix that?</p>