title: Subsetting the last entry of the day in a data frame
body: <pre><code>&gt; dataframe
  
      time        value                date
  1      2           28 2013-07-08 15:13:35
  2      4            8 2013-07-08 15:14:06
  3      7            2 2013-07-08 15:43:05
  4      8           25 2013-07-09 16:30:41
  5     11           12 2013-07-09 19:04:40
  6     14           41 2013-07-09 19:20:14
  7     18           12 2013-07-10 00:39:04
  8     22           12 2013-07-10 08:27:02
  </code></pre>
  
  <p>Does anyone know of a way to subset the last entry of each day in a dataframe?
  i.e. obtain:</p>
  
  <pre><code>    time        value                date
  3      7            2 2013-07-08 15:43:05
  6     14           41 2013-07-09 19:20:14
  8     22           12 2013-07-10 08:27:02
  </code></pre>
  
  <p>Many thanks !</p>