the no of days in my datagridview is increasing

Private Sub ComputeLedger()
        Dim interest = txtIntRate1.Text
        Dim interest2 = txtIntRate2.Text
        Dim princiapalLoan = txtBalAmortize.Text
        Dim matValue = txtMoAmort1.Text
        Dim matValue2 = txtMoAmort2.Text
        Dim dblMat As Double = Decimal.Parse(matValue, Globalization.NumberStyles.Currency)
        matValue = dblMat.ToString(Format("###,###,###.#0", "Standard"))
        Dim dblMat2 As Double = Decimal.Parse(matValue2, Globalization.NumberStyles.Currency)
        matValue2 = dblMat2.ToString(Format("###,###,###.#0", "Standard"))

        Dim effDate = dtpValueDate.Value
        Dim dueDate = dtpFirstDueDate.Value
        Dim firstdueDate = DateTimePicker1.Value
        Dim secondeffDate = DateTimePicker2.Value

        Dim i As Integer
        Dim weekInt As Integer
        Dim weekInt3 As Integer
        Dim weekInt2 As Integer = Val(txtTerms2.Text)
        Dim durations As Integer = Val(txtTerms1.Text)
        Dim durations2 As Integer = Val(lblTotalTerms.Text)
        'DataGridView1.Rows.Clear()

        Dim ndays As Integer
        ndays = DateDiff("d", firstdueDate, secondeffDate)

        Dim areturnv As Double '= (princiapalLoan / durations2)
        Dim dblret2 As Double = Decimal.Parse(areturnv, Globalization.NumberStyles.Currency)
        Dim ainterest As Double '= ((princiapalLoan * (interest2)) * (ndays / 365))
        Dim dblint2 As Double = Decimal.Parse(ainterest, Globalization.NumberStyles.Currency)
        Dim princ As Double '= (matValue - ainterest)
        Dim dblprin2 As Double = Decimal.Parse(princ, Globalization.NumberStyles.Currency)
        Dim tolamt = (princiapalLoan)
        Dim dblamt2 As Double = Decimal.Parse(tolamt, Globalization.NumberStyles.Currency)
        Dim tolamt3 As Double '= (princiapalLoan - princ)
        Dim dblamt3 As Double = Decimal.Parse(tolamt3, Globalization.NumberStyles.Currency)
        'Dim tolamt4 As Double '= (princiapalLoan - princ)
        Dim dblamt4 As Integer 'Double = Decimal.Parse(tolamt4, Globalization.NumberStyles.Currency)

       Do While durations < durations2
            durations += 1
            weekInt3 += 1

            secondeffDate = firstdueDate.AddMonths(weekInt3)
            If secondeffDate.DayOfWeek = DayOfWeek.Saturday Then
                secondeffDate = secondeffDate.AddDays(2)
            ElseIf secondeffDate.DayOfWeek = DayOfWeek.Sunday Then
                secondeffDate = secondeffDate.AddDays(1)
            End If

            Select Case dblamt4
                Case "01" 'January"
                    dblamt4 = "31"
                Case "02" 'February"
                    dblamt4 = "28"
                Case "03" 'March"
                    dblamt4 = "31"
                Case "04" 'April"
                    dblamt4 = "30"
                Case "05" 'May"
                    dblamt4 = "31"
                Case "06" 'June"
                    dblamt4 = "30"
                Case "07" 'July"
                    dblamt4 = "31"
                Case "08" 'August"
                    dblamt4 = "31"
                Case "09" 'September"
                    dblamt4 = "30"
                Case "10" 'October"
                    dblamt4 = "31"
                Case "11" 'November"
                    dblamt4 = "30"
                Case "12" 'December"
                    dblamt4 = "31"
            End Select

'my problem is in code. 
            ndays = DateDiff(DateInterval.Day, firstdueDate.AddDays(dblamt4), firstdueDate.AddMonths(weekInt3))
            dblret2 = ((princiapalLoan * (interest2)) * (ndays / 365))

            dblint2 = dblMat2 - dblret2
            dblamt3 = dblamt3 - dblint2

            DataGridView1.Rows.Add(secondeffDate.ToString("MM/dd/yyyy"), _
                                    dblMat2.ToString(Format("###,###,###.#0", "Standard")), _
                                    ndays, _
                                    dblret2.ToString(Format("###,###,###.#0", "Standard")), _
                                    dblint2.ToString(Format("###,###,###.#0", "Standard")), _
                                    dblamt3.ToString(Format("###,###,###.#0", "Standard")))
        Loop


    End Sub