title: converting time in factor format into timestamp
body: <p>I've downloaded tweets in json format, converted it into csv, and read it into R.  The existing time stamps are in factor format as shown below.  How should I convert it into a timestamp that can be plotted against?</p>
  
  <pre><code>[1] Fri May 09 07:55:12 +0000 2014 Fri May 09 07:55:12 +0000 2014 Fri May 09 07:55:12 +0000 2014
  [4] Fri May 09 07:55:12 +0000 2014 Fri May 09 07:55:12 +0000 2014 Fri May 09 07:55:12 +0000 2014
  516 Levels: Fri May 09 07:55:12 +0000 2014 ... Fri May 09 09:15:07 +0000 2014
  </code></pre>