title: Error in POSIXlt
body: <p>Date in my data is in YYYY-dd-mm format.<code>2015-01-07,2015-01-08</code> are some sample dates.I want to extract week number from this date. I used following code</p>
  
  <pre><code>week_number &lt;- as.POSIXlt(data_tmp2$my_date)
  data_tmp2$week_number=strftime(data_tmp2,format="%W")
  </code></pre>
  
  <p>After running the second line,I'm getting following error</p>
  
  <pre><code>Error in as.POSIXlt.default(x, tz = tz) : 
  do not know how to convert 'x' to class “POSIXlt”
  </code></pre>
  
  <p>Can you please help me to solve this issue?</p>