title: Convert Date to POSIXct
body: <p>Why does the Date below change to "2014-07-07" when converted to POSIXct?</p>
  
  <pre><code>Sys.setenv(TZ='America/Sao_Paulo')
  d &lt;- as.Date("2014-07-08", format="%Y-%m-%d")
  d
  [1] "2014-07-08"
  as.POSIXct(d)
  [1] "2014-07-07 21:00:00 BRT"
  </code></pre>