title: Control POSIX time difference output in R
body: <p>In <code>R</code> I have two strings:</p>
  
  <pre><code>t &lt;- c("2010-01-01 00:01:02", "2010-01-01 00:02:02")
  </code></pre>
  
  <p>which I convert to POSIX datetimes:</p>
  
  <pre><code>dt &lt;- as.POSIXct(t)
  </code></pre>
  
  <p>Taking the difference of the two <code>dt[2] - dt[1]</code> gives:</p>
  
  <pre><code>Time difference of 1 mins
  </code></pre>
  
  <p>Cool.  But how do I force the time difference to be in, say, seconds?</p>