title: Convert Gregorian date to Julian date
body: <p>I have a df like so:</p>
  
  <pre><code>ImageDate   Year   Month   Day
  20020506    2002   05      06
  20030605    2003   06      05
  20040201    2002   02      01
  </code></pre>
  
  <p>and I want to add a new column which converts the dates into julian dates.  I am pretty sure I need the module <code>datetime</code> to do this, but I looked through the docs and didn't find anything that was apparent to me on how to do this.</p>