[excel] Automatic date update in a cell when another cell's value changes (as calculated by a formula)

You could fill the dependend cell (D2) by a User Defined Function (VBA Macro Function) that takes the value of the C2-Cell as input parameter, returning the current date as ouput.

Having C2 as input parameter for the UDF in D2 tells Excel that it needs to reevaluate D2 everytime C2 changes (that is if auto-calculation of formulas is turned on for the workbook).

EDIT:

Here is some code:

For the UDF:

    Public Function UDF_Date(ByVal data) As Date

        UDF_Date = Now()

    End Function

As Formula in D2:

=UDF_Date(C2)

You will have to give the D2-Cell a Date-Time Format, or it will show a numeric representation of the date-value.

And you can expand the formula over the desired range by draging it if you keep the C2 reference in the D2-formula relative.

Note: This still might not be the ideal solution because every time Excel recalculates the workbook the date in D2 will be reset to the current value. To make D2 only reflect the last time C2 was changed there would have to be some kind of tracking of the past value(s) of C2. This could for example be implemented in the UDF by providing also the address alonside the value of the input parameter, storing the input parameters in a hidden sheet, and comparing them with the previous values everytime the UDF gets called.

Addendum:

Here is a sample implementation of an UDF that tracks the changes of the cell values and returns the date-time when the last changes was detected. When using it, please be aware that:

• The usage of the UDF is the same as described above.

• The UDF works only for single cell input ranges.

• The cell values are tracked by storing the last value of cell and the date-time when the change was detected in the document properties of the workbook. If the formula is used over large datasets the size of the file might increase considerably as for every cell that is tracked by the formula the storage requirements increase (last value of cell + date of last change.) Also, maybe Excel is not capable of handling very large amounts of document properties and the code might brake at a certain point.

• If the name of a worksheet is changed all the tracking information of the therein contained cells is lost.

• The code might brake for cell-values for which conversion to string is non-deterministic.

• The code below is not tested and should be regarded only as proof of concept. Use it at your own risk.

  Public Function UDF_Date(ByVal inData As Range) As Date
  
      Dim wb As Workbook
      Dim dProps As DocumentProperties
      Dim pValue As DocumentProperty
      Dim pDate As DocumentProperty
      Dim sName As String
      Dim sNameDate As String
  
      Dim bDate As Boolean
      Dim bValue As Boolean
      Dim bChanged As Boolean
  
      bDate = True
      bValue = True
  
      bChanged = False
  
  
      Dim sVal As String
      Dim dDate As Date
  
      sName = inData.Address & "_" & inData.Worksheet.Name
      sNameDate = sName & "_dat"
  
      sVal = CStr(inData.Value)
      dDate = Now()
  
      Set wb = inData.Worksheet.Parent
  
      Set dProps = wb.CustomDocumentProperties
  
  On Error Resume Next
  
      Set pValue = dProps.Item(sName)
  
      If Err.Number <> 0 Then
          bValue = False
          Err.Clear
      End If
  
  On Error GoTo 0
  
      If Not bValue Then
          bChanged = True
          Set pValue = dProps.Add(sName, False, msoPropertyTypeString, sVal)
      Else
          bChanged = pValue.Value <> sVal
          If bChanged Then
              pValue.Value = sVal
          End If
      End If
  
  On Error Resume Next
  
      Set pDate = dProps.Item(sNameDate)
  
      If Err.Number <> 0 Then
          bDate = False
          Err.Clear
      End If
  
  On Error GoTo 0
  
      If Not bDate Then
          Set pDate = dProps.Add(sNameDate, False, msoPropertyTypeDate, dDate)
      End If
  
      If bChanged Then
          pDate.Value = dDate
      Else
          dDate = pDate.Value
      End If
  
  
      UDF_Date = dDate
   End Function
  

Make the insertion of the date conditional upon the range.

This has an advantage of not changing the dates unless the content of the cell is changed, and it is in the range C2:C2, even if the sheet is closed and saved, it doesn't recalculate unless the adjacent cell changes.

Adapted from this tip and @Paul S answer

Private Sub Worksheet_Change(ByVal Target As Range)
 Dim R1 As Range
 Dim R2 As Range
 Dim InRange As Boolean
    Set R1 = Range(Target.Address)
    Set R2 = Range("C2:C20")
    Set InterSectRange = Application.Intersect(R1, R2)

  InRange = Not InterSectRange Is Nothing
     Set InterSectRange = Nothing
   If InRange = True Then
     R1.Offset(0, 1).Value = Now()
   End If
     Set R1 = Nothing
     Set R2 = Nothing
 End Sub