Conversion failed when converting date and/or time from character string

protected void Button1_Click(object sender, EventArgs e)
{
    DateTime date1 = DateTime.ParseExact(Txt_refd.Text, "yyyy-MM-dd", null);
    DateTime date2 = DateTime.ParseExact(Txt_out.Text, "yyyy-MM-dd", null);

    using (var command = new SqlCommand())
    {
        command.CommandText = "INSERT INTO FZMain (ID, NAME, ChassisNO, CarModel, LastOwner, Payment, Cases, Refference, RefferenceDate, OutDate, FamilyRegister) " +
            "VALUES (@ID, @NAME, @ChassisNO, @CarModel, @LastOwner, @Payment, @Cases, @Refference, @RefferenceDate, @OutDate, @FamilyRegister)";
        command.Connection = con;
        command.Parameters.AddWithValue("@ID", Txt_ID.Text);
        command.Parameters.AddWithValue("@NAME", Txt_name.Text);
        command.Parameters.AddWithValue("@ChassisNO", Txt_chasses.Text);
        command.Parameters.AddWithValue("@CarModel", Txt_model.Text);
        command.Parameters.AddWithValue("@LastOwner", Txt_owner.Text);
        command.Parameters.AddWithValue("@Payment", Txt_payment.Text);
        command.Parameters.AddWithValue("@Cases", Drop_case.SelectedValue.ToString());
        command.Parameters.AddWithValue("@Refference", Txt_ref);
        command.Parameters.AddWithValue("@RefferenceDate", date1);
        command.Parameters.AddWithValue("@OutDate", date2);
        command.Parameters.AddWithValue("@FamilyRegister", Txt_femreg.Text);
        // Carry on from this point
    }
}

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