NSDate / NSDateFormatter - 仅存储时间，而不是日期？

I've been looking around but I haven't seen anything that addresses this so I'm hoping someone can help clear this up for me. What I am trying to do is use an NSDate variable(in core data) to store a time, not date and time, but just time in the format HH:MM:SS; I've been looking around but I haven't seen any