title: AttributeError: Can only use .dt accessor with datetimelike values
body: <p>Hi I am using pandas to convert a column to month. 
  When I read my data they are objects:</p>
  
  <pre><code>Date           object
  dtype: object
  </code></pre>
  
  <p>So I am first making them to date time and then try to make them as months:</p>
  
  <pre><code>import pandas as pd
  file = '/pathtocsv.csv'
  df = pd.read_csv(file, sep = ',', encoding='utf-8-sig', usecols= ['Date', 'ids'])    
  df['Date'] = pd.to_datetime(df['Date'])
  df['Month'] = df['Date'].dt.month
  </code></pre>
  
  <p>Also if that helps: </p>
  
  <pre><code>In [10]: df['Date'].dtype
  Out[10]: dtype('O')
  </code></pre>
  
  <p>So, the error I get is like this:</p>
  
  <pre><code>/Library/Frameworks/Python.framework/Versions/2.7/bin/User/lib/python2.7/site-packages/pandas/core/series.pyc in _make_dt_accessor(self)
     2526             return maybe_to_datetimelike(self)
     2527         except Exception:
  -&gt; 2528             raise AttributeError("Can only use .dt accessor with datetimelike "
     2529                                  "values")
     2530 
  
  AttributeError: Can only use .dt accessor with datetimelike values
  </code></pre>
  
  <p>EDITED: </p>
  
  <p>Date columns are like this: </p>
  
  <pre><code>0         2014-01-01         
  1         2014-01-01         
  2         2014-01-01         
  3         2014-01-01         
  4         2014-01-03       
  5         2014-01-03         
  6         2014-01-03         
  7         2014-01-07         
  8         2014-01-08         
  9         2014-01-09 
  </code></pre>
  
  <p>Do you have any ideas?
  Thank you very much! </p>