Option Explicit

Sub Calculate()
    Dim dataTable() As Variant
    Dim resultTable() As Variant
    Dim rowsNumber As Integer
    Dim columsNumber As Integer
    Dim i As Integer
    Dim mon As Integer
    Dim days_in_month As Integer
    Dim days_in_year As Integer
    Dim days_to_deposit As Integer
    Dim days_from_deposit_to_fund As Integer
    Dim days_from_fund_to_end As Integer
    Dim intrest_to_deposit As Double
    Dim intrest_from_deposit_to_fund As Double
    Dim intrest_from_fund_to_end As Double
    Dim intrest As Double
    Dim rate As Double

    rowsNumber = ActiveWorkbook.Sheets(1).Cells(Rows.Count, "C").End(xlUp).Row - 1
    columsNumber = ActiveWorkbook.Sheets(1).Cells(2, Columns.Count).End(xlToLeft).Column - 2

    ReDim Preserve dataTable(1 To rowsNumber, 1 To columsNumber)
    ReDim Preserve resultTable(1 To rowsNumber, 1 To columsNumber)

    dataTable = ActiveWorkbook.Sheets(1).Range(Cells(2, 3), Cells(rowsNumber + 1, columsNumber + 2)).Value

    mon = MonthNumber()
    days_in_month = Day(DateSerial(Year(Now()), mon + 1, 1) - 1)
    days_in_year = DateDiff("d", DateSerial(Year(Now()), 1, 1), DateSerial(Year(Now()) + 1, 1, 1))

    For i = 1 To UBound(dataTable, 1)
        rate = dataTable(i, 5) / days_in_year
        If CDate(dataTable(i, 4)) > CDate(dataTable(i, 7)) Then
            days_to_deposit = DateDiff("d", DateSerial(Year(Now()), mon, 1), dataTable(i, 7)) + 1
            days_from_deposit_to_fund = DateDiff("d", dataTable(i, 7), dataTable(i, 4))
            days_from_fund_to_end = DateDiff("d", dataTable(i, 4), DateSerial(Year(Now()), mon, days_in_month))

            intrest_to_deposit = dataTable(i, 1) * rate * days_to_deposit
            intrest = dataTable(i, 2) + intrest_to_deposit - dataTable(i, 6)

            If intrest < 0 Then
                resultTable(i, 2) = 0
                resultTable(i, 1) = dataTable(i, 1) + intrest
            Else
                resultTable(i, 2) = intrest
                resultTable(i, 1) = dataTable(i, 1)
            End If

            intrest_from_deposit_to_fund = resultTable(i, 1) * rate * days_from_deposit_to_fund
            resultTable(i, 2) = resultTable(i, 2) + intrest_from_deposit_to_fund

            intrest_from_fund_to_end = (resultTable(i, 1) + dataTable(i, 3)) * rate * days_from_fund_to_end
            resultTable(i, 2) = resultTable(i, 2) + intrest_from_fund_to_end

        End If

        resultTable(i, 3) = dataTable(i, 3)
        resultTable(i, 4) = "-"
        resultTable(i, 5) = dataTable(i, 5)
        resultTable(i, 6) = 0
        resultTable(i, 7) = "-"
    Next i

    ActiveWorkbook.Sheets(1).Range(Cells(2, 3), Cells(rowsNumber + 1, columsNumber + 2)) = resultTable

End Sub

Function MonthNumber() As Integer
    Dim month_from_sheet As String
    Dim mon As String

    month_from_sheet = ActiveWorkbook.Sheets(1).Range("K1").Value

    Select Case month_from_sheet
    Case "Styczeñ"
        mon = 1
    Case "Luty"
        mon = 2
    Case "Marzec"
        mon = 3
    Case "Kwiecieñ"
        mon = 4
    Case "Maj"
        mon = 5
    Case "Czerwiec"
        mon = 6
    Case "Lipiec"
        mon = 7
    Case "Sierpieñ"
        mon = 8
    Case "Wrzesieñ"
        mon = 9
    Case "Pa¿dziernik"
        mon = 10
    Case "Listopad"
        mon = 11
    Case "Grudzieñ"
        mon = 12
    End Select

    MonthNumber = mon
End Function