title: Convert an ff POSIXct vector to an ff numeric vector
body: <p>I am trying to convert an <strong>ff</strong> vector with <em>POSIXct</em> entries to an <strong>ff</strong> <em>numeric</em> vector, containing the respective number of seconds since the origin 01-01-1970.</p>
  
  <p><code>x = as.ff(as.POSIXct(c("2014-06-30 00:01:27 BST", "2014-06-30 00:02:17 BST")))</code></p>
  
  <p>The 'natural' <code>as.numeric(x)</code> does not work, yielding: <code>numeric(0)</code>.</p>
  
  <p><code>as.ff(as.numeric(x[]))</code> works, yielding</p>
  
  <p><code>ff (open) double length=2 (2)
         [1]        [2] 
  1398898887 1398898937</code></p>
  
  <p>which is the desirable outcome. Albeit, it involves the ram object <code>x[]</code>. Is there a way to achieve the above result employing ff objects only (i.e. not intermediating the ram equivalent of the ff vector)?</p>