title: what does "property with 'weak' attribute must be of object type" really mean?
body: <p>I have this in my interface:</p>
  
  <pre><code>@property (nonatomic, weak) NSTimeInterval *timeStamp;
  </code></pre>
  
  <p>Which my logic told me, I need a time stamp object, that only is going to be used by this class within the context of its instantiation, so "weak" seemed to be logical to me-- but XCode tells me "property with 'weak' attribute must be of object type"...  If I just do:</p>
  
  <pre><code>@property (nonatomic) NSTimeInterval *timeStamp;
  </code></pre>
  
  <p>Then the error goes away, but I am not sure I understand why...</p>