title: customize highcharts tooltip to show datetime
body: <p>I'm developing a project that is expected to show this graph: <a href="http://jsfiddle.net/Kc23N/">http://jsfiddle.net/Kc23N/</a></p>
  
  <p>When I click a point (tooltip) I want to show a date understandable, not the value in milliseconds.</p>
  
  <p>I think needs to change this piece of code:</p>
  
  <pre><code>tooltip: {
        headerFormat: '&lt;b&gt;{series.name}&lt;/b&gt;&lt;br&gt;',
        pointFormat: '{point.x} date, {point.y} Kg',                        
  }
  </code></pre>
  
  <p>how do I do? any kind help is appreciated.</p>
  
  <p>Thanks</p>