title: timestamp conversion error in R: Error in unclass(x)/86400 : non-numeric argument to binary operator
body: <p>My dataset is read from csv and it is quite big. I have many fields of timestamp attributes, all imported as character variables. When trying to convert these to dates and calculate difference in seconds, I ran into such error in only 1 of the timestamp variables:</p>
  
  <p>Date imported:</p>
  
  <pre><code>ID  clicked_at  bounced_at  retrieved_at    dropped_at
  323232  4/23/16 02:13   4/23/16 02:14   4/23/16 02:16   4/23/16 02:16
  323582  3/29/16 19:00   3/29/16 19:00   3/29/16 19:03   3/29/16 19:04
  235302  6/21/16 11:56   6/21/16 11:56   6/21/16 12:01   6/21/16 12:01
  </code></pre>
  
  <p>Code used to convert timestamp:</p>
  
  <pre><code>test$bounce_speed &lt;- as.POSIXct(test$bounced_at) - as.POSIXct(test$clicked_at)
  </code></pre>
  
  <p>Error got: </p>
  
  <pre><code>Error in unclass(x)/86400 : non-numeric argument to binary operator
  </code></pre>
  
  <p>After testing, I believe the problem is in <code>retrieved_at</code> variable. I used the same code to calculate lag between other variables here, all ran okay. But just by eye-balling, I could not find anything wrong in <code>retrieved_at</code>, and my data size is huge. My guess is some records have irregular formats, but how to fix this here?</p>