title: Pandas datetime column to ordinal
body: <p>I'm trying to create a new Pandas dataframe column with ordinal day from a datetime column:</p>
  
  <pre><code>import pandas as pd
  from datetime import datetime
  
  print df.ix[0:5]
                                date
  file                              
  gom3_197801.nc 2011-02-16 00:00:00
  gom3_197802.nc 2011-02-16 00:00:00
  gom3_197803.nc 2011-02-15 00:00:00
  gom3_197804.nc 2011-02-17 00:00:00
  gom3_197805.nc 2011-11-14 00:00:00
  
  df['date'][0].toordinal()
  
  Out[6]:
  734184
  
  df['date'].toordinal()
  
  ---------------------------------------------------------------------------
  AttributeError                            Traceback (most recent call last)
  &lt;ipython-input-7-dbfd5e8b60f0&gt; in &lt;module&gt;()
  ----&gt; 1 df['date'].toordinal()
  
  AttributeError: 'Series' object has no attribute 'toordinal'
  </code></pre>
  
  <p>I guess this is a basic question, but I've struggled reading docs for last 30 minutes. </p>
  
  <p>How can I create an ordinal time column for my dataframe?</p>