We're making use of the Focused/UnFocused event. What we've found is the DatePicker will have the value you've just picked all set and ready for you to use in the event when UnFocused occurs. The TimePicker does not - it shows the TimeSpan value as it was before you picked a date.
How do I get this raised as a bug? I've managed to work around the problem by using the PropertyChanged event (e.PropertyName == "Time") luckily.