Date time error

Question

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

button_Print.Visible = true;

        //comboBox_emp.Visible = false;
        string connectionString = "Data Source=.\\sqlexpress;Initial Catalog=db-ub;Integrated Security=True";
        string sql = ("SELECT * FROM Visitors WHERE Day_In='" + Convert.ToDateTime(dateTimePicker1.Value) + "'");
        SqlConnection connection = new SqlConnection(connectionString);
        connection.Open();
        sCmd = new SqlCommand(sql, connection);
        sAdpt = new SqlDataAdapter(sCmd);
        sBldr = new SqlCommandBuilder(sAdpt);
        sDs = new DataSet();
        sAdpt.Fill(sDs, "Visitors");
        sTable = sDs.Tables["Visitors"];
        connection.Close();
        dataGridView_UB.DataSource = sDs.Tables["Visitors"];
        dataGridView_UB.ReadOnly = true;
        dataGridView_UB.SelectionMode = DataGridViewSelectionMode.FullRowSelect;
    }