title: r as.POSIXct get NA
body: <p>Why did I get all NAs when converting timestamps below using as.POSIXct?</p>
  
  <pre><code>&gt; head(tmp$timestamp_utc)
  [1] Fri Jul 03 00:15:00 EDT 2015 Fri Jul 03 00:45:00 EDT 2015 Fri Jul 03 01:15:00 EDT 2015 Fri Jul 03 01:45:00 EDT 2015 Fri Jul 03 02:15:00 EDT 2015
  [6] Fri Jul 03 02:45:00 EDT 2015
  &gt; tmp$timestamp_utc&lt;- as.POSIXct(tmp$timestamp_utc, "%m/%d/%Y %H:%M:%S", tz="GMT")
  &gt; head(tmp$timestamp_utc)
  [1] NA NA NA NA NA NA
  </code></pre>
  
  <p>UPDATE: 
          1. The initial problem is the format was wrong which was pointed out the some friends below. Correct format will not generate NA
          2. Then I find out EDT was not converted to GMT, though "GMT" is added in as.POSIXCT. Using with_tz solves this issue.</p>
  
  <pre><code>&gt; as.POSIXct("Fri Jul 03 00:15:00 EDT 2015", format="%a %b %d %H:%M:%S EDT %Y", tz="GMT")
  [1] "2015-07-03 00:15:00 GMT"
  &gt; with_tz(as.POSIXct("Fri Jul 03 00:15:00 EDT 2015", format="%a %b %d %H:%M:%S EDT %Y"),"GMT")
  [1] "2015-07-03 04:15:00 GMT"
  </code></pre>