Помогите с ошибкой, “Не удалось привести тип объекта ”System.DataTime“ к типу ”System.String"

    MySqlConnection conn = new MySqlConnection(connStr);
                    conn.Open();

                        string sql = $"select subscribe from dle_users where email = '{textBoxUserEmail.Text}' and password = '{sspass}' ";
                        MySqlCommand command = new MySqlCommand(sql, conn);
                        command.Prepare();
                        command.ExecuteNonQuery();
                    if (textBoxUserEmail.Text == (string)command.ExecuteScalar())
                    {
                        DateTime d1;
                        try
                        {
                            string date = command.ExecuteScalar().ToString();
                            var client = new TcpClient("time.nist.gov", 13);
                            using (var streamReader = new StreamReader(client.GetStream()))
                            {
                                var response = streamReader.ReadToEnd();
                                var utcDateTimeString = response.Substring(7, 17);
                                d1 = DateTime.ParseExact(utcDateTimeString, "yy-MM-dd HH:mm:ss", CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal);
                            }
                            DateTime d2 = Convert.ToDateTime(date);
                            TimeSpan time = d2 - d1;
                            int days = time.Days;
                            if (days > 0)
                            {
                                passlIsPass = true;
                                if (conn != null && conn.State != ConnectionState.Closed)
                                    conn.Close();
                            }
                            if (days < 0)
                            {
                                DialogResult result = MessageBox.Show("Продлите подписку !", "Внимание", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                            }
                        }
                        catch (Exception ex)
                        {
                            DialogResult result = MessageBox.Show("Ошибка", "Внимание", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                        }
                    }

                    else
                    {
                        passlIsPass = false;
                        DialogResult result = MessageBox.Show("Логин или пароль введен неверно", "Внимание", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                    }