设置DateTimePicker初始值为空，按delete或backSpace时显示空值

    private void FrmUserEdit_Load(object sender, EventArgs e)
        {
            DtpDepartureDate.Format = DateTimePickerFormat.Custom;
            DtpDepartureDate.CustomFormat = " ";

            DtpDateOfJoining.Format = DateTimePickerFormat.Custom;
            DtpDateOfJoining.CustomFormat = null;
        }

        private void DtpDepartureDate_ValueChanged(object sender, EventArgs e)
        {
            DtpDepartureDate.CustomFormat = null;
        }


        private void DtpDepartureDate_KeyUp(object sender, KeyEventArgs e)
        {
            if (e.KeyCode ==Keys.Delete || e.KeyCode == Keys.Back)
            {
                DtpDepartureDate.CustomFormat = " ";
            }
        }