private DateTime date;
        public DateTime Date
        {
            get
            {
                return date;
            }
            set
            {
                if (value != date)
                {
                    date = value;
                    NotifyPropertyChanged("Date");
                    NotifyPropertyChanged("DateToString");//主要是这里
                }
            }
        }

        public String DateToString
        {
            get
            {
                return Date.ToString("yyyy-MM-dd");
            }
        }