title: Create the timestamp of an hour ago
body: <p>I would like to create a timestamp by a variable of hour.</p>
  
  <p>for example:
  current timestamp is:</p>
  
  <pre><code>2015-09-29 14:00:00
  </code></pre>
  
  <p>my variable is : <code>$hour = 2</code></p>
  
  <p>new timestamp that I want:</p>
  
  <pre><code>2015-09-29 16:00:00
  </code></pre>