title: convert local dateTime to UTC in R
body: <p>How can I convert local DateTime in the following format <code>"12/31/2014 6:42:52 PM"</code> to UTC in R? I tried this </p>
  
  <p><code>as.POSIXct(as.Date("12/31/2014 6:42:52 PM", format="%m/%d/%Y %H:%M:%S"),tz="UTC")</code></p>
  
  <p>but it doesn't seem to be valid. </p>