如何通过增加小时数来改变NSDate的日期？

I have an app firing off UILocalNotifications every hour, which will stop at the time the user selected in a date picker. I pre set 23 local notifications by adding hour components by one. The problem is, when the hour passes 12:00 AM the day stays in the same day and does not change. How do I have the NSDate change the day when the hour reaches 12:00 AM? Thanks for the help. I have an app firing off UILocalNotifications e