title: convert date to unix time in R
body: <p>I want to convert "2013-09-16" into unix time. </p>
  
  <p>I found <a href="https://stackoverflow.com/questions/13456241/convert-unix-epoch-to-date-object-in-r">Convert UNIX epoch to Date object in R</a> but I need to do the reverse of that.</p>
  
  <p>Thanks!</p>