title: Summing rows by month in R
body: <p>So I have a data frame that has a date column, an hour column and a series of other numerical columns. Each row in the data frame is 1 hour of 1 day for an entire year.</p>
  
  <p>The data frame looks like this:</p>
  
  <pre><code>          Date  Hour  Melbourne  Southern  Flagstaff
  1   2009-05-01     0          0         5         17
  2   2009-05-01     2          0         2          1
  3   2009-05-01     1          0        11          0
  4   2009-05-01     3          0         3          8
  5   2009-05-01     4          0         1          0
  6   2009-05-01     5          0        49         79
  7   2009-05-01     6          0       425        610
  </code></pre>
  
  <p>The hours are out of order because this is subsetted from another data frame.</p>
  
  <p>I would like to sum the values in the numerical columns by month and possibly by day. Does anyone know how I can do this?</p>