title: R converting twitter created_at string to time
body: <p>I am trying to convert the created_at string but it returns NA</p>
  
  <pre><code>as.POSIXct("Tue Jun 07 23:27:12 +0000 2016", format="%a %b %d %H:%M:%S +0000 %Y", tz="GMT")
  [1] NA
  </code></pre>
  
  <p>Any idea what's going wrong, seems fairly straightforward!</p>