title: Converting date in R with as.POSIXct() returns NA
body: <p>I am working through the book 'R for dummies', 2nd Edition, John Wiley &amp; Sons, Inc.
  In chapter 6 you try adding time information into the vector but the shown code is not working:</p>
  <p>Book:</p>
  <pre><code>apollo &lt;- &quot;July 20, 1969, 20:17:39&quot;
  apollo.fmt &lt;- &quot;%B %d, %Y, %H:%M:%S&quot;
  xct &lt;- as.POSIXct(apollo, format = apollo.fmt, tz = &quot;UTC&quot;)
  xct
  #&gt; [1] &quot;1969-07-20 20:17:39 UTC&quot;
  </code></pre>
  <p>My try:</p>
  <pre><code>apollo &lt;- &quot;July 20, 1969, 20:17:39&quot;
  apollo.fmt &lt;- &quot;%B %d, %Y, %H:%M:%S&quot;
  xct &lt;- as.POSIXct(apollo, format = apollo.fmt, tz = &quot;UTC&quot;)
  xct
  #&gt; [1] NA
  </code></pre>
  <p>Anyone knows what I am doing wrong, and what should be the right code?</p>