title: obtain hour from DateTime vector
body: <p>I have a DateTime vector within a data.frame where the data frame is made up of 8760 observations representing hourly intervals throughout the year e.g.</p>
  
  <pre><code>2010-01-01 00:00
  2010-01-01 01:00
  2010-01-01 02:00
  2010-01-01 03:00
  </code></pre>
  
  <p>and so on.</p>
  
  <p>I would like to create a data.frame which has the original DateTime vector as the first column and then the hourly values in the second column e.g.</p>
  
  <pre><code>2010-01-01 00:00          00:00
  2010-01-01 01:00          01:00
  </code></pre>
  
  <p>How can this be achieved? </p>