NSNull versus (null)

OK, the book says that an NSNull instance (=null) can be used to represent "nothingness," e.g. in a NSMutableArray. But it looks like this will be the value of an NSDate instance variable that you declare but fail to implement (by which I mean that you fail to assign any value to). At least, its descriptor will return "(null)."

But then it looks like I was wrong; it's just a mere case of homonymy: (null) is not null. Even though my unassigned NSDate instance entryDate was described as "(null)" by its description method, it's not an NSNull instance, since

[entryDate isEqual:[NSNull null]]

returns 0. And NSNull has only one instance, so every null would be "equal" to each other. Right?

So what's this "(null)"??! NSDate's documentation doesn't say anything about it, and neither does NSObject's.