title: How to group a data.frame by date?
body: <p>I have a .csv file in the following format:</p>
  
  <pre><code>Date       ,     Time  , Value
  1899-01-01 ,  4:00:00  ,    1
  1899-01-01 ,  4:01:00  ,    2
  1899-01-01 ,  4:02:00  ,    3
  1899-01-01 ,  4:03:00  ,    4
  1899-01-01 ,  4:04:00  ,    5
  1900-08-22 , 22:00:00  ,  101
  1900-08-22 , 22:01:00  ,  102
  2013-08-29 ,  4:00:00  , 1000
  2013-02-29 ,  4:02:00  , 1001
  2013-02-29 ,  4:03:00  , 1002
  </code></pre>
  
  <p>Is it possible to <code>group by date</code> to produce a <code>data.table</code> in the the following format:</p>
  
  <pre><code>Date      , Vector(variable length)
  1899-02-28, c(1,2,3,4,5)
  1900-08-22, c(101,102)
  1900-08-22, c(1000,1001,1002)
  </code></pre>
  
  <p>This is the best that I have so far (after a day of attempts):</p>
  
  <pre><code>raw &lt;- read.csv(pathName, header = TRUE, stringsAsFactors = FALSE)
  groupedByDate &lt;- split(raw, raw$Date)
  </code></pre>
  
  <p>However, this seems to produce a very wide table with one column for each date, which is not very close to what I want.</p>