title: NSNumber and NSTimeInterval
body: <p>I am trying to get the system time in milliseconds.
   For that I have declared:</p>
  
  <pre><code> NSNumber *createdTimeInMilliSec; //in class declaration
  </code></pre>
  
  <p>and in one of my instance functions, I doing:</p>
  
  <pre><code> self.createdTimeInMilliSec= ([NSDate timeIntervalSinceReferenceDate]*1000); //ERROR: incompatible type for argument 1 of 'setCreatedTimeInMilliSec:'
  </code></pre>
  
  <p><code>timeIntervalSinceReferenceDate</code> returns in <code>NSTimeInterval</code>, so how to convert that into <code>NSNumber</code>? Or what I am doing wrong?</p>