mjc,
If you add this code to the Sheet module for the worksheet you need "watched", it should do the trick. Basically,
every time you edit a cell in Col B, Col A gets the current system date.
Private Sub Worksheet_Change(ByVal Target As Range)
Dim MaxRows As Long
Dim cel As Range
MaxRows = Me.Rows.Count
If Not Intersect(Me.Range("b2:b" & MaxRows), Target) Is Nothing Then
Application.EnableEvents = False
For Each cel In Intersect(Me.Range("b2:b" & MaxRows), Target).Cells
If cel <> "" Then
Cells(cel.Row, "a") = Date
Else
Cells(cel.Row, "a").ClearContents
End If
Next
Application.EnableEvents = True
End If
End Sub
Patrick