title: Set units of difference between datetime objects
body: <p>The <code>diff</code> command returns the differences between dates in a vector of dates in the R date format. I'd like to control the units that are returned, but it seems like they are automatically determined, with no way to control it w/ an argument. Here's an example:   </p>
  
  <pre><code> &gt; t = Sys.time() 
   &gt; diff(c(t, t + 1))
   Time difference of 1 secs
  </code></pre>
  
  <p>And yet: </p>
  
  <pre><code>&gt; diff(c(t, t+10000))
  Time difference of 1.157407 days
  </code></pre>
  
  <p>The "time delta" object has a units attribute, but it seems silly to write a bunch of conditionals to coerce everything into days, seconds etc.   </p>