title: Time difference in R: POSIXct error
body: <p>I am using this code to calculate time difference.</p>
  
  <pre><code>ctym &lt;- lapply(G, function(x) x[k,2])
  y &lt;- lapply(G, function(x) x[j,2])
  t1 &lt;- c(y,ctym)
  dt1 &lt;- as.POSIXct(t1)
  dtyme &lt;- difftime(dt1[2],dt1[1],units = "mins")
  </code></pre>
  
  <p>Output for t1 is:</p>
  
  <pre><code> [[1]]
        time_occurred 
  "2013-04-01 20:27:18"
  
  [[2]]
      time_occurred 
  "2013-04-01 20:27:48"
  </code></pre>
  
  <p>Error I am getting is:
  Error in as.POSIXct.default(t1) : 
    do not know how to convert 't1' to class “POSIXct”</p>
  
  <p>What's wrong here? </p>