Problem with NSSortDescriptor using custom comparator via selector

I want to use a sortdescriptor with a custom comparator

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] 
    initWithKey:@"object.dateTime" 
    ascending:YES 
    selector:@selector(compareObject:toObject:)];

(the key is a ManagedObject)

Comparator method:

- (NSComparisonResult)compareObject:(id)date1 toObject:(id)date2 {
    NSComparisonResult comparisonResult;
    // Complex comparator contents
    return comparisonResult;
}

However, I get an error: 'NSInvalidArgumentException ', reason: '-[__NSDate compareObject:toObject:]: unrecognized selector sent .....

What am I doing wrong? The comparator works if I use it in a block, but I need it to work via a selector. I can't find any example code or clear documentation on how to use comparators via a selector (for iOS 3.x.x). The documentation talks about comparing to self, but I've tried to incorporate the compare method in object, but that didn't work either.

Who can point me to my problem or to some example code as to how to use this via a selector?

Note: The comparator itself is not a simple date comparison. There is a lot more going on in there.