That sample I mentioned does show how you can edit a DateTime object and enter both the date and time portions of it. The drop-down is just a MonthCalendar (no time) right now.
However that being said, we have started designing one of our next products: Editors for WPF. That will include a more robust implementation of a date/time picker, that will support things like part-based editing, up/downs, customizable drop-downs, etc. So that would probably give you what you'd like to see.