title: Year-month-week expression
body: <p>I have a data written in specific expression. To simplify the data, here is the example I made:</p>
  <pre><code>df&lt;-data.frame(date=c(2012034,2012044,2012051,2012063,2012074),
                 math=c(100,100,23,46,78))
  </code></pre>
  <p><code>2012034</code> means 4th week of march,2012. Likewise <code>2012044</code> means 4th week of April,2012. I was trying to make the values of <code>date</code> expressing some order. The reason why I have to do this is because when I don't change them to time expressions, x axis of the scatter plot looks really weird.</p>
  <p>My goal is this:
  Find the oldest date in <code>date</code> column and name it as 1. In this case, <code>2012034</code> should be <code>1</code>. Next, find the second oldest date in <code>date</code> column and  calculate how many weeks passed after that <code>date</code>. The second oldest date in <code>date</code> is <code>2012044</code>.So, 5 weeks after the oldest date <code>2012034</code>. So it should be changed as 1+5=6. So, likewise, I want to number the <code>date</code> to indicate how many weeks have passed since the oldest <code>date</code></p>