In your interface create a public property or method for retrieving the entire date. This method or property will then create the date using your interface fields. What you have done sounds like a user friendly interface as clear separation of the two is logical.
public DateTime GetDateTime(){
return new DateTime(DateControl.Year,DateControl.Month,DateControl.Day,TimeControl.Hour,TimeControl.Minute);
}
As an example this could be a possible method on your interface which can be consumed!
Subquently you could have a set method which takes a datetime and you then split of the values and bind the two controls. :-)
Andrew