title: calculating time difference in R
body: <p>I have a data with more that 3 million records having start.time and end.time as two of the variables. The first 10 obs are as follows:</p>
  
  <pre><code>   start.date start.time   end.date end.time
  1  2012-07-13   15:01:32 2012-07-13 15:02:42
  2  2012-07-05   18:26:31 2012-07-05 18:27:19
  3  2012-07-14   20:23:21 2012-07-14 20:24:11
  4  2012-07-29   16:09:54 2012-07-29 16:10:48
  5  2012-07-21   14:58:32 2012-07-21 15:00:17
  6  2012-07-04   15:36:31 2012-07-04 15:37:11
  7  2012-07-22   18:28:31 2012-07-22 18:28:50
  8  2012-07-09   21:08:42 2012-07-09 21:09:02
  9  2012-07-05   09:44:52 2012-07-05 09:45:05
  10 2012-07-02   18:50:47 2012-07-02 18:51:38
  </code></pre>
  
  <p>I need to calculate the difference between start.time and end.time.</p>
  
  <p>I used the following code:</p>
  
  <pre><code>mbehave11$diff.time &lt;- difftime(mbehave11$end.time, mbehave11$start.time, units="secs")
  </code></pre>
  
  <p>But I am getting this error:</p>
  
  <pre><code>Error in as.POSIXlt.character(x, tz, ...) : 
    character string is not in a standard unambiguous format
  In addition: Warning messages:
  1: In is.na.POSIXlt(strptime(xx, f &lt;- "%Y-%m-%d %H:%M:%OS", tz = tz)) :
    Reached total allocation of 1535Mb: see help(memory.size)
  </code></pre>