[CODE]
 Dim a As Date = Date.Parse(TextBox2.Text)
        Dim b As Date = Date.Parse(TextBox3.Text)
        Dim source2 As String = WebConfigurationManager.ConnectionStrings("ConnectionString").ConnectionString
        Dim sql2 As String = "select SUM(DATEDIFF(day, dal, al)+1) AS Total FROM Tab1 WHERE codfisc = @codfisc and Dal >=@Dal and Al <=@al"
        Dim conn2 As New SqlConnection(source2)
        conn2.Open()
        Dim cmd2 As New SqlCommand(sql2, conn2)
        cmd2.Parameters.AddWithValue("@Codfisc", Me.GridView1.SelectedRow.Cells(8).Text)
        cmd2.Parameters.AddWithValue("@Dal", a)
        cmd2.Parameters.AddWithValue("@Al", b)
        Dim dr2 As SqlDataReader
        dr2 = cmd2.ExecuteReader()
        While dr2.Read()
            TextBox4.Text = dr2("Total").ToString()
        End While
        conn2.Close()
[/CODE]