title: "AttributeError: Can only use .dt accessor with datetimelike values" when trying to extract year from a date column
body: <p>I got a dateframe which there is one column 'brth_dt' and its type is datetime64[ns].I want to extract the age of persons,however when input:
  all_df['brth_dt'].dt.year or<br>
  all_df['age'] = (pd.datetime.today().year - all_df['brth_dt'].dt.year) </p>
  
  <p>coming up an error:can only use .dt accessor with datetimelike values</p>
  
  <p>brth_dt columns are like this:</p>
  
  <pre><code>    brth_date
  1   14Oct1978
  2   21Aug1970
  3   06Jan1980
  4   09Mar1992
  </code></pre>
  
  <p>any advice?thanks!</p>