title: Pandas to_datetime ValueError: Unknown string format
body: <p>I have a column in my (pandas) dataframe:</p>
  
  <pre><code>data['Start Date'].head()
  type(data['Start Date'])
  Output:
  1/7/13
  1/7/13
  1/7/13
  16/7/13
  16/7/13
  &lt;class 'pandas.core.series.Series'&gt;
  </code></pre>
  
  <p>When I convert it into a date format (as follows) I am getting the error <strong>ValueError: Unknown string format</strong></p>
  
  <pre><code>data['Start Date']= pd.to_datetime(data['Start Date'],dayfirst=True)
  ...
  ...
  /Library/Python/2.7/site-packages/pandas/tseries/tools.pyc in _convert_listlike(arg, box, format, name)
      381                 return DatetimeIndex._simple_new(values, name=name, tz=tz)
      382             except (ValueError, TypeError):
  --&gt; 383                 raise e
      384 
      385     if arg is None:
  
  ValueError: Unknown string format
  </code></pre>
  
  <p>What am I missing here?</p>