problem using function isdate & not isdate by vba

Hi Speed

Try this (and the attached file). I've commented the code so you see what I've changed and why. Western users should note that the sheet uses right-to-left direction (which affects offset addressing):: 

Private Sub Worksheet_change(ByVal Target As Range)

' quit unless a single cell in changed in G
If Target.Cells.CountLarge > 1 Or Intersect(Target, Range("g:g")) Is Nothing Then Exit Sub

'set earliest date you'll accept (as a String)
Earliest = "01/01/2000"

' check if a date after that was added
If Not IsDate(Target.Value) Or Target.Value < DateValue(Earliest) Then
    MsgBox ("Please enter a valid date (after " & Earliest & ") in " & Target.Address(0, 0) & ". Example: m/dd/yyyy")

    Else
    ' if so
    Target.Offset(0, -2) = "done" '### offset is -2 since sheet uses right-to-left driection, not left-to-right
    MsgBox "The date has been changed in cell " & Target.Address(0, 0) & vbCrLf & "The date is " & Target.Value, vbExclamation

End If

End Sub

Note that I've used Target.Address(0, 0) which sets both row and column absolute addressing ($) to 0 (=False). I also set an Earliest date (which you can change) to prevent errors, since you could type 67 in column G and it would be displayed/ seen by VBA as a date (17 March 1990).

Hope this is what you want.