title: converting a pandas date to week number
body: <p>I would like to extract a week number from data in a pandas dataframe.</p>
  
  <p>The date format is datetime64[ns]</p>
  
  <p>I have normalized the date to remove the time from it</p>
  
  <pre><code>df['Date'] = df['Date'].apply(pd.datetools.normalize_date)
  </code></pre>
  
  <p>so the date now looks like - 2015-06-17 in the data frame column</p>
  
  <p>and now I like to convert that to a week number.</p>
  
  <p>Thanks in advance</p>