title: How to change column in Matrix in R?
body: <pre><code>Test
  N        Date Column2  
  1  2015-07-15      56   
  2  2015-04-17      45
  </code></pre>
  
  <p>How can I change a column with dates to a column with real numbers, I want to change each date to a number of month of date, for example <code>2015-07-15</code> change to <code>7</code>, <code>2014-05-13</code> to <code>5</code>.</p>