Calculate the Date of an Item Based on the Date of a Similar Item in the Previous Month

Quite by accident I have just found someone also having problems with the PrevSheet UDF, not only that they had a solution that works! It is at

HTML Code: 

(not sure if I got that right). Maybe my computer took pity on me and went looking on its own! ;-)

the code is:

Function PrevSheet(Rg As Range)
'accounts for more than one workbook open
'and has hidden sheets
'Bob Phillips  October 4, 2009
Dim n As Variant
    With Application.Caller.Parent
        n = .Index
        Do
            If n = 1 Then
                PrevSheet = CVErr(xlErrRef)
                Exit Do
            ElseIf TypeName(.Parent.Sheets(n - 1)) <> "Chart" And _
                .Parent.Sheets(n - 1).Visible = xlSheetVisible Then
                PrevSheet = .Parent.Sheets(n - 1).Range(Rg.Address).Value
                Exit Do
            End If
            n = n - 1
        Loop
    End With
End Function

I have done a quick check and it seems to work as is. I just hope there is nothing lurking in some dark dank corner waiting to pop up and bite me in the bum!