The issue is that you need to assign self.title the correct date value.
The line:
self.title = NSDate()
is assigning self.title to a new instance of NSDate(), while you want to assign it to the formatted date that you have above, convertedDate, which is also aString`.
Additionally, NSDate() is of type NSDate and not of type String. If you converted it to a String it would be unformatted.
So really, you need the line to be:
self.title = convertedDate
solved Adding a self populating date into the Navigation Bar Title [closed]