Convert formula to value once formula has calculated - excel

Question

I'm using the following VBA module to create a Timestamp UDF, which stamps the date once the referenced cell reads "Done":
Function Timestamp(Reference As Range)
If Reference.Value = "Done" Then
Timestamp = Format(Date, "ddd dd mmm")
Else
Timestamp = ""
End If
End Function
The date stays the same even after refreshing / closing and opening the workbook as long as the referenced cell still reads "Done"; however if someone accidentally changes the referenced cell then the date is reset.
I need a VBA code to convert the formula to value once it has calculated, so the date will always stay the same. The solution needs to be automatic rather than manual and I can't enable iterative formulas on this workbook because it's used by multiple users. Any help much appreciated!

Answer 1

You can use the Worksheet Change event for that:
Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
Dim AffectedCells As Range
Set AffectedCells = Intersect(Target, Me.Range("A:A")) ' Range A:A is the range we observe for 'done'
If AffectedCells Is Nothing Then Exit Sub
Dim Cell As Range
For Each Cell In AffectedCells
If Cell.Value = "done" Then
Dim UpdateTimestamp As Boolean
UpdateTimestamp = True
If Range("B" & Cell.Row).Value <> vbNullString Then
UpdateTimestamp = MsgBox("Timestamp exists do you want to update it?", vbQuestion + vbYesNo) = vbYes
End If
If UpdateTimestamp Then
Me.Range("B" & Cell.Row).Value = Format$(Date, "ddd dd mmm")
End If
End If
Next Cell
End Sub
// Edit according comment:
If you want to check multiple ranges for different things you need to slightly change your code:
Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
Dim Cell As Range
Dim ObservedRangeA As Range
Set ObservedRangeA = Intersect(Target, Me.Range("A:A")) ' Range A:A is the range we observe for 'done'
If Not ObservedRangeA Is Nothing Then
For Each Cell In ObservedRangeA
If Cell.Value = "done" And Range("B" & Cell.Row).Value = vbNullString Then
Me.Range("B" & Cell.Row).Value = Format$(Date, "ddd dd mmm")
End If
Next Cell
End If
Dim ObservedRangeB As Range
Set ObservedRangeB = Intersect(Target, Me.Range("C:C")) ' Range C:C is the range we observe for ""
If Not ObservedRangeB Is Nothing Then
For Each Cell In ObservedRangeB
If Cell.Value = "" And Range("B" & Cell.Row).Value = vbNullString Then
Me.Range("B" & Cell.Row).Value = Format$(Date, "ddd dd mmm")
End If
Next Cell
End If
End Sub