title: Cannot create survival object: error "Time variable is not numeric"
body: <p>I try to create a survival object using this command.</p>
  
  <pre><code>Surv(date1, date2, event=status)
  </code></pre>
  
  <p>It resulted in error "Time variable is not numeric".
  Note that class of "date1" and "date2" is Date.</p>
  
  <p>Do I need to coerce Date objects into numeric before creating survival objects?
  I think this doesn't make any sense.</p>