title: split or subset data into 30 minute intervals
body: <p>I have a data frame of the following form:</p>
  
  <pre><code>Temp    Depth      Light   x   time date                   time.at.depth
  104     18.59      -2.7    27 21:38 2012-06-20             4
  109     18.59      -2.7    27 22:02 2012-06-20             5
  110     18.75      -4.0    27 22:07 2012-06-20             5
  113     18.91      -2.7    27 22:21 2012-06-20             4
  114     18.91      -4.0    27 22:26 2012-06-20             5
  115     18.91      -2.7    27 22:31 2012-06-20             5
  117     18.91      -2.7    27 22:40 2012-06-20             4
  118     18.75      -4.0    27 22:45 2012-06-20             5
  119     18.75      -2.7    27 22:50 2012-06-20             5
  121     18.59      -4.0    27 22:59 2012-06-20             4
  122     18.75      -2.7    27 23:04 2012-06-20             5
  123     18.75      -4.0    27 23:09 2012-06-20             5
  126     18.59      -2.7    27 23:23 2012-06-20             5
  127     18.59      -2.7    27 23:28 2012-06-20             5
  128     18.59      -4.0    27 23:33 2012-06-20             5
  133     18.75      -4.0    27 23:57 2012-06-20             5
  136     18.59      -4.0    27 00:11 2012-06-20             5
  138     18.59      -2.7    27 00:21 2012-06-20             5
  140     18.91      -2.7    27 00:30 2012-06-20             5
  </code></pre>
  
  <p>I would like to subset the data into 30 minute bins. Is there an easy way to do this? I looked at the <code>split()</code> function but it appears that things need to be in a numeric format for that to work, I have time in <code>POSIXct</code> format. Any ideas would be greatly appreciated.</p>