时间选择
DateTimePicker.ShowUpDown = true;
DateTimePicker.CustomFormat = "hh:mm";
DateTimePicker.Format = System.Windows.Forms.DateTimePickerFormat.Custom;
设置为空
dtp.Format = DateTimePickerFormat.Custom;
dtp.CustomFormat = " ";
dtp.Value = DateTime.FromOADate(0);
dtp.Text = string.Empty;