The code I gave you has some problems when the
workbook is closed. Also, technically it is not a
user designed function. The following code placed
in a standard module should do what you want.
It will update every time the workbook is recalculated.
To use it, in a cell just enter ...
= timesaved()

Jim Cone

'--------------------------
Function TimeSaved()
Application.Volatile
Dim strPath As String
strPath = ActiveWorkbook.Path
TimeSaved = "Last saved: " & FileDateTime(strPath)
End Function