title: Finding the date for a given week number
body: <p>I am trying to do some date math based on the week number of a given year. For example:</p>
  
  <pre><code>date = Date.today # Monday, March 5, 2012
  puts date.cwyear  # 2012
  puts date.cweek   # 10 (10th week of 2012)
  </code></pre>
  
  <p>Now that I know what the current week is, I want to figure out what the next week and previous week are. I need to take the year (2012) and the week number (10) and turn it back into a date object so I can calculate the value for the next/previous week. How can I do this?</p>