title: Convert week number to date
body: <p>I have a data frame in R with the week of the year that I would like to convert to a date. I know I have to pick a year and a day of the week so I am fixing those values at 2014 and 1.  Converting this to a date seems simple: </p>
  
  <pre><code>as.Date(paste(2014,df$Week,1,sep=""),"%Y%U%u")
  </code></pre>
  
  <p>But this code only works if week is greater than 9. Week 1 - 9 returns NA. If I change the week to 01,02,03... it still returns NA. </p>
  
  <p>Anyone see what I am missing?</p>