title: R select row by year and month
body: <p>I have à dataFrame like </p>
  
  <pre><code>head(JournalBeart)
          date     VTemperature
  1 2012-03-08 10.2938450704225
  2 2012-03-09 10.6240559440559
  3 2012-03-10 12.4791398601399
  4 2012-03-11 14.3567482517483
  5 2012-03-12 15.9947622377622
  6 2012-03-13 16.1366433566434
  </code></pre>
  
  <p>And I want to select line by month and year. I would select all rows that are for the months between April and June of 2012 
  I have find for month, </p>
  
  <pre><code>month(JournalBeart$date) %in% 3:9
  </code></pre>
  
  <p>how to integrate the month and year</p>
  
  <p>Thanks!</p>