Interest Calculation

I have a macro that formats a spreadsheet to show outstanding invoices, grouped and subtotalled by month. To add to this I need VBA code that will use the subtotals to calculate interest on overdue accounts.

Interest becomes due a calender month after the month in which the invoice is dated. So for example a January invoice would start to accrue interest on 1st March.

Below is the subtotals code (sadly the totals don't adjust if data is added or removed but perhaps that is another question for another day.)

Dim LastRow As Long
Dim NextMonth As String
Dim R As Long
Dim Rng As Range
Dim SubAmount As Currency
Dim ThisMonth As String
Dim TotalAmount As Currency
Dim Wks As Worksheet

Set Wks = Worksheets("Reconciliation")
LastRow = Wks.Cells(Rows.Count, "A").End(xlUp).Row
Set Rng = Wks.Range(Cells(2, "A"), Cells(LastRow, "D"))