title: Use a static variable inside an Objective-C category
body: <p>I want to be able to do the following:</p>
  
  <pre><code>NSString *hoursAndMinutes = [myNSDateObject formattedTime]; // eg. 10:25
  </code></pre>
  
  <p>Now I can create a category on the <code>NSDate</code> class but since the creation of <code>NSDateFormatter</code> has proven to be quite costly I would like to use a single instance of my <code>NSDateFormatter</code>. Is this a contradiction to the notion of categories or is there a neater way to achieve this?</p>