Calculating the prior month given the current month

Hello everyone. I am currently coding a project which requires information from a prior month. What I have is a message box which prompts the user to enter the date: dd-mmm-YYYY format. That value is then stored as a variable deontoed fDate. I would like to create a sub macro which takes fDate and"subtracts a month" and create a file which will be named after that, for instance if the current month is 30-jan-2010, then a file will be created and named 31-dec-2009. My line of thought was "fdate -1", but that returns an error. Please see the code below, specificaly Sub CreateFiles (). Any help is greatly appreciated.

Option Explicit

    Dim fDate  As String
    Dim fPath  As String

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Sub CreateFolder()

    Dim Fldr   As String
    Dim ErrBuf As String

    fDate = Application.InputBox("Enter a date in the format shown:", "Date to add...", Format(Date, "DD-MMM-YYYY"))
    If fDate = "False" Then Exit Sub

    fPath = "L:\"

    On Error GoTo ErrorHandler
        
        Fldr = fPath & fDate & "_157"
        MkDir Fldr
        
        Fldr = fPath & fDate & "_157\" & "157_Reports"
        MkDir Fldr
        
        Fldr = fPath & fDate & "_157\" & "Support_Summaries"
        MkDir Fldr

        Fldr = fPath & fDate & "_157\" & "157_Reports\" & "Roll_forward_wTA"
        MkDir Fldr
        
        Fldr = fPath & fDate & "_157\" & "157_Reports\" & "Roll_forward_12m"
        MkDir Fldr
        
        Fldr = fPath & fDate & "_157\" & "157_Reports\" & "Terminated"
        MkDir Fldr
        
        Fldr = fPath & fDate & "_157\" & "157_Reports\" & "L3_IDA"
        MkDir Fldr
        
        Fldr = fPath & fDate & "_157\" & "157_Reports\" & "IBRD_L3"
        MkDir Fldr
        
        Fldr = fPath & fDate & "_157\" & "105_Reports"
        MkDir Fldr
    
    If Len(ErrBuf) > 0 Then MsgBox "The following folders already existed:" & vbLf & vbLf & ErrBuf
    
    Exit Sub
    
ErrorHandler:
    ErrBuf = ErrBuf & vbLf & Fldr
    Resume Next
    
End Sub

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Sub MoveFilesFolder2Folder()
    Dim fso
    Dim sfol As String
    Dim dfol As String

    sfol = "L:\157_Support_Summaries"
    dfol = fPath & fDate & "_157\" & "Support_Summaries"

    Set fso = CreateObject("Scripting.FileSystemObject")
        
        On Error Resume Next
        
        If Not fso.FolderExists(sfol) Then
        MsgBox sfol & " is not a valid folder/path.", vbInformation, "Invalid Source"

        ElseIf Not fso.FolderExists(dfol) Then
        MsgBox dfol & " is not a valid folder/path.", vbInformation, "Invalid Destination"
        
        Else
        fso.CopyFile (sfol & "\*.*"), dfol ' Change "\*.*" to "\*.xls" to move Excel Files only
        
        End If

        If Err.Number = 53 Then MsgBox "File not found"
End Sub

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Sub CreateFiles()

    Dim sPath1      As String
    Dim sPath2      As String
    Const sFileOut1 As String = "105_version1.xlsm"
    Const sFileOut2 As String = "CDS.xlsm"
    
    sPath1 = fPath & fDate & "_157\105_Reports\"
    sPath2 = fPath & fDate & "_157\Support_Summaries\"
    
    Workbooks.Add
    With ActiveSheet
        .Name = fDate & "_105_v1"
        .Parent.SaveAs Filename:=sPath1 & sFileOut1, _
                       FileFormat:=xlOpenXMLWorkbookMacroEnabled, _
                       CreateBackup:=False
        .Parent.Close
    End With
    
    Workbooks.Add
    With ActiveSheet
        .Name = fDate - 1 & "_CDS" <------------------------------------- Error
        .Parent.SaveAs Filename:=sPath2 & sFileOut2, _
                       FileFormat:=xlOpenXMLWorkbookMacroEnabled, _
                       CreateBackup:=False
        .Parent.Close
    End With
    
End Sub