Create timestamp formula for Excel

Here's a full-fledged solution that will allow you to monitor change-dates of different ranges. Note, this uses a function from Chip Pearson's tools for using arrays in VBA and function from a StackOverflow answer by user Thomas.

The basic idea is that a global array, in which the addresses of all monitored ranges (past or present) are stored with their most recent update dates, allows the function and the Worksheet_Change Sub to interact. The Worksheet_Change Sub updates this array by checking the changed range against all stored ranges. The function looks for the monitored range in the array and returns the stored change-date if it's found. Otherwise, it will return today's date (which will then be added to the array).

Also, to prevent timestamps being lost when the workbook is closed and the array of timestamps is deallocated, the array must be written to a sheet on the Workbook_Close event, and then rewritten to the array on the Workbook_Open event.

In a module, paste the following code.

Public funcInstances() As Variant

Public Function MOD_DATE_OF(monitor As Range)
Application.Volatile True
Dim i As Long
Dim tmpArray() As Variant

If Not IsDimensioned(funcInstances) Then
    ReDim funcInstances(1 To 1, 1 To 2) As Variant
    funcInstances(1, 1) = monitor.Address
    funcInstances(1, 2) = Date
Else
    For i = 1 To UBound(funcInstances, 1)
        If funcInstances(i, 1) = monitor.Address Then
            MOD_DATE_OF = Format(funcInstances(i, 2), "yyyy-mm-dd")
            Exit Function
        End If
    Next i
    tmpArray = ExpandArray(funcInstances, 1, 1, "")
    Erase funcInstances
    funcInstances = tmpArray
    funcInstances(UBound(funcInstances, 1), 1) = monitor.Address
    funcInstances(UBound(funcInstances, 1), 2) = Date
End If
MOD_DATE_OF = Format(funcInstances(UBound(funcInstances, 1), 2), "yyyy-mm-dd")
End Function

'ExpandArray() is the work of Chip Pearson.  Code copied from http://www.cpearson.com/excel/vbaarrays.htm
Function ExpandArray(Arr As Variant, WhichDim As Long, AdditionalElements As Long, _
        FillValue As Variant) As Variant
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
' ExpandArray
' This expands a two-dimensional array in either dimension. It returns the result
' array if successful, or NULL if an error occurred. The original array is never
' changed.
' Parameters:
' --------------------
' Arr                   is the array to be expanded.
'
' WhichDim              is either 1 for additional rows or 2 for
'                       additional columns.
'
' AdditionalElements    is the number of additional rows or columns
'                       to create.
'
' FillValue             is the value to which the new array elements should be
'                       initialized.
'
' You can nest calls to Expand array to expand both the number of rows and
' columns. E.g.,
'
' C = ExpandArray(ExpandArray(Arr:=A, WhichDim:=1, AdditionalElements:=3, FillValue:="R"), _
'    WhichDim:=2, AdditionalElements:=4, FillValue:="C")
' This first adds three rows at the bottom of the array, and then adds four
' columns on the right of the array.
'
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
Dim Result As Variant
Dim RowNdx As Long
Dim ColNdx As Long
Dim ResultRowNdx As Long
Dim ResultColNdx As Long
Dim NumRows As Long
Dim NumCols As Long
Dim NewUBound As Long

Const ROWS_ As Long = 1
Const COLS_ As Long = 2


''''''''''''''''''''''''''''
' Ensure Arr is an array.
''''''''''''''''''''''''''''
If IsArray(Arr) = False Then
    ExpandArray = Null
    Exit Function
End If

'''''''''''''''''''''''''''''''''
' Ensure the dimension is 1 or 2.
'''''''''''''''''''''''''''''''''
Select Case WhichDim
    Case 1, 2
    Case Else
        ExpandArray = Null
        Exit Function
End Select

''''''''''''''''''''''''''''''''''''
' Ensure AdditionalElements is > 0.
' If AdditionalElements  < 0, return NULL.
' If AdditionalElements  = 0, return Arr.
''''''''''''''''''''''''''''''''''''
If AdditionalElements < 0 Then
    ExpandArray = Null
    Exit Function
End If
If AdditionalElements = 0 Then
    ExpandArray = Arr
    Exit Function
End If

NumRows = UBound(Arr, 1) - LBound(Arr, 1) + 1
NumCols = UBound(Arr, 2) - LBound(Arr, 2) + 1

If WhichDim = ROWS_ Then
    '''''''''''''''
    ' Redim Result.
    '''''''''''''''
    ReDim Result(LBound(Arr, 1) To UBound(Arr, 1) + AdditionalElements, LBound(Arr, 2) To UBound(Arr, 2))
    ''''''''''''''''''''''''''''''
    ' Transfer Arr array to Result
    ''''''''''''''''''''''''''''''
    For RowNdx = LBound(Arr, 1) To UBound(Arr, 1)
        For ColNdx = LBound(Arr, 2) To UBound(Arr, 2)
            Result(RowNdx, ColNdx) = Arr(RowNdx, ColNdx)
        Next ColNdx
    Next RowNdx
    '''''''''''''''''''''''''''''''
    ' Fill the rest of the result
    ' array with FillValue.
    '''''''''''''''''''''''''''''''
    For RowNdx = UBound(Arr, 1) + 1 To UBound(Result, 1)
        For ColNdx = LBound(Arr, 2) To UBound(Arr, 2)
            Result(RowNdx, ColNdx) = FillValue
        Next ColNdx
    Next RowNdx
Else
    '''''''''''''''
    ' Redim Result.
    '''''''''''''''
    ReDim Result(LBound(Arr, 1) To UBound(Arr, 1), UBound(Arr, 2) + AdditionalElements)
    ''''''''''''''''''''''''''''''
    ' Transfer Arr array to Result
    ''''''''''''''''''''''''''''''
    For RowNdx = LBound(Arr, 1) To UBound(Arr, 1)
        For ColNdx = LBound(Arr, 2) To UBound(Arr, 2)
            Result(RowNdx, ColNdx) = Arr(RowNdx, ColNdx)
        Next ColNdx
    Next RowNdx
    '''''''''''''''''''''''''''''''
    ' Fill the rest of the result
    ' array with FillValue.
    '''''''''''''''''''''''''''''''
    For RowNdx = LBound(Arr, 1) To UBound(Arr, 1)
        For ColNdx = UBound(Arr, 2) + 1 To UBound(Result, 2)
            Result(RowNdx, ColNdx) = FillValue
        Next ColNdx
    Next RowNdx

End If
''''''''''''''''''''
' Return the result.
''''''''''''''''''''
ExpandArray = Result

End Function

'IsDimensioned() is the work of StackOverflow user @Thomas.  Code copied from https://stackoverflow.com/a/5480690/657668
Public Function IsDimensioned(vValue As Variant) As Boolean
    On Error Resume Next
    If Not IsArray(vValue) Then Exit Function
    Dim i As Integer
    i = UBound(vValue)
    IsDimensioned = Err.Number = 0
End Function

In the appropriate Worksheet module, paste the following code.

Private Sub Worksheet_Change(ByVal Target As Range)
Application.EnableEvents = False
Dim j As Long
If IsDimensioned(funcInstances) Then
    For j = 1 To UBound(funcInstances, 1)
        If Not Intersect(Target, Range(funcInstances(j, 1))) Is Nothing Then
            funcInstances(j, 2) = Date
        End If
    Next j
    Me.Calculate
End If
Application.EnableEvents = True
End Sub

Finally, in the ThisWorkbook module, paste the following code:

Private Sub Workbook_BeforeClose(Cancel As Boolean)
If IsDimensioned(funcInstances) Then
    Application.ScreenUpdating = False
    'Store array on a new temporary and hidden worksheet.
    Dim tmpS As Worksheet, tmpR As Range
    Set tmpS = Worksheets.Add
    tmpS.Name = "TEMP Record of Timestamps"
    tmpS.Visible = xlSheetHidden
    Set tmpR = tmpS.Range("A1:B1").Resize(UBound(funcInstances, 1), 2)
    tmpR.Value = funcInstances
    ThisWorkbook.Save
    Application.ScreenUpdating = True
End If
End Sub

Private Sub Workbook_Open()
Dim ws As Worksheet, tstamps As Range
Dim wsfound As Boolean
wsfound = False
Application.ScreenUpdating = False
For Each ws In ThisWorkbook.Worksheets
    If ws.Name = "TEMP Record of Timestamps" Then
        wsfound = True
        Exit For
    End If
Next ws
If wsfound Then
    Set tstamps = ws.UsedRange
    funcInstances = tstamps.Value
    Application.DisplayAlerts = False
    ws.Delete
    Application.DisplayAlerts = True
End If
Application.ScreenUpdating = True
End Sub

NOTE for anyone who stumbles across this page: Many of the comments are about previous, incomplete solutions, so don't be confused by them.

Create timestamp formula for Excel

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