The only reason I'm not calling this a blocker is that the same behavior exists in Sakai 10.
If you take an existing date and bring up the picker, you can change the hour, minute, or day, and things work fine. But if you change the month or year, it seems in the picker like you've changed them, but they aren't saved. You have to pick a day before the new values will save. What seems to be happening is that if you change the year or month, it clear the day, but not if you change the minute or how.
I think this is a bug, because you get into a situation where things like fine in the picker (the fact that the date is no longer set is nearly invisible), but they don't actually work.