Seems to be that the date picker is missing from WinRT.
As Xerax said, the common solution is Callisto (I lied, Callisto doesn't have a working datepicker yet). I can't find a working control on Google that's free, you might have to use a number picker with some date logic.
Does anyone know the logic behind why they didn't include a DatePicker control? Seems pretty retarded on the face of it to omit something as frequently used as a date picker.