title: Calculating days of week given a week number
body: <p>Given a week number, e.g. <code>date -u +%W</code>, how do you calculate the days in that week starting from Monday?</p>
  
  <p>Example rfc-3339 output for week 40:</p>
  
  <pre><code>2008-10-06
  2008-10-07
  2008-10-08
  2008-10-09
  2008-10-10
  2008-10-11
  2008-10-12
  </code></pre>