Odd issue with custom Excel VBA resetting values

Question

I'm doing a few things at once which I believe may be causing issues. I've tested this VBA in Office 2013 and Office 2016 under Windows 10.

I have a multitude of worksheets, each of which is titled based on the month and year (ex: "November 2018", "December 2018", etc.). I'm using VBA to do two (separate) things:

1. Get the Active Worksheet's name
2. Traverse a previous worksheet's data

VBA Code:

Public Function RelSheet(iPos As Integer, zRange As String)

    'Relative Worksheet Reference Facility
    'eg: =RelSheet(-1,"A3") = Cell A3 in Previous (Left) WSheet
    'eg: =RelSheet(1,"A3") = Cell A3 in Next (Right) WSheet
    'eg: "#Error" when reference does not exist
    'eg: Can do maths =RelSheet(1,"A3")*2

    Dim shtActive As Worksheet
    Application.Volatile True
    Set shtActive = Application.Caller.Worksheet
    On Error GoTo BadSheetReference
    RelSheet = Sheets(shtActive.Index + iPos).Range(zRange).Value

    GoTo ExitFunction

BadSheetReference:
    RelSheet = "#Error"

ExitFunction:
End Function

Function TabName()
  TabName = ActiveSheet.Name
End Function

In my worksheets, I'm tallying a summation month over month until a near year is created, then the tally begins again at 0 (or whatever value Jannuary contains). Cell C8 is the current month's value, cell C9 is the summation of the previous month's value (C9 of the previous worksheet) + the current worksheet cell's value in C8. The formula for that cell (C9) is as follows: ==IF(ISNUMBER(SEARCH("January", TabName())), C8, RelSheet(-1, "C9")+C8)

Unfortunately, once a worksheet name does contain the text of "January", all previous worksheets are also reverted to a sum of 0. I believe it is related to the RelSheet function and essentially recursively checking itself, but when I step through the code logically on paper, I don't see how it's doing this. My current workaround is just to manually set the value of the particular cell in a January worksheet to 0, and continue the formula on successive spreadsheets.

Answer 1

Couple of suggestions:

Public Function RelSheet(iPos As Integer, zRange As String)

    'Relative Worksheet Reference Facility
    'eg: =RelSheet(-1,"A3") = Cell A3 in Previous (Left) WSheet
    'eg: =RelSheet(1,"A3") = Cell A3 in Next (Right) WSheet
    'eg: "#Error" when reference does not exist
    'eg: Can do maths =RelSheet(1,"A3")*2

    Dim shtActive As Worksheet
    Application.Volatile True
    Set shtActive = Application.Caller.Worksheet
    On Error GoTo BadSheetReference
    '##added workbook qualifier
    RelSheet = ThisWorkbook.Sheets(shtActive.Index + iPos).Range(zRange).Value

    GoTo ExitFunction

BadSheetReference:
    RelSheet = CVErr(xlErrRef)

ExitFunction:
End Function

Function TabName()
  '## not ActiveSheet
  TabName = Application.Caller.Parent.Name
End Function