title: add time for timestamp
body: <p>What is the best method for adding time to a timestamp? 
  I have in a database</p>
  
  <p><strong>2011-10-09 10:29:23</strong></p>
  
  <p>and I would like add to this 20 days. How can I do that?
  I need an example for:</p>
  
  <ol>
  <li>20 hours</li>
  <li>20 days</li>
  </ol>