title: Convert UNIX epoch to Date object
body: <p>I'm plotting and performing calculations on uniformly distributed time series. The timestamps are currently stored as integers representing the number of seconds since the UNIX epoch (e.g. <code>1352068320</code>), but <code>Date</code> objects seem more appropriate for plotting. How can I do the conversion?</p>
  
  <p>I've read <code>?Date</code>, <code>?as.Date</code> and <code>??epoch</code>, but seem to have missed that information.</p>