title: How to convert a date to YYYYDDD?
body: <p>I can't figure out how to turn Sys.Date() into a number in the format YYYYDDD. Where DDD is the day of the year, i.e. Jan 1 would be 2016001 Dec 31 would be 2016365</p>
  
  <pre><code>Date &lt;- Sys.Date()  ## The Variable Date is created as 2016-01-01
  SomeFunction(Date)  ## Returns 2016001
  </code></pre>