I have hesitated to mention dates as a separate topic because I have a vested interest. However, numerous threads have mentioned dates as a pain that people would like to sort out.
The JDK dates have numerous issues, including the Date class, with lots of deprecated methods and should have been immutable, plus the Calendar class, with its poor internal design that can be highly unreliable.
My interest? I look after Joda Time, a rewrite of dates and times (http://joda-time.sourceforge.net). The trouble is that there is a case to argue that libraries can be better written and more responsive when outside the JDK.