title: Can't init static calendar in initialize method of NSDate category
body: <p>I've created a helpers category for NSDate (NSDate+Helpers). Here is a caching for the calendar object I'm going to use inside the category implementation:</p>
  
  <pre><code>static NSCalendar* currentCalendar = nil;
  @implementation NSDate (Helpers)
  
  +(void)initialize{
      currentCalendar = [NSCalendar currentCalendar];
  }
  @end
  </code></pre>
  
  <p>The problem is that my application doesn't starts anymore. I see only black screen. I would like to know what I am doing wrong. You can get same problem on clean test project.</p>