title: Converting unix seconds in milliseconds to POSIXct/POSIXlt
body: <p>Why do I see a difference when I convert a unix timestamp to datetime object in R?</p>
  
  <pre><code>&gt; as.POSIXlt(1268736919, origin="1970-01-01", tz="America/New_York")
  [1] "2010-03-16 06:55:19 EDT"
  
  &gt; as.POSIXct(1268736919, origin="1970-01-01", tz="America/New_York")
  [1] "2010-03-16 11:55:19 EDT"
  </code></pre>
  
  <p>The result from POSIXlt is actually correct.</p>
  
  <p>Also, is there a way to do this conversion without specifying the origin?</p>
  
  <p>Thanks</p>