I did say "In theory"!
If you need the last save time / year you can query the Doc Props
Code:
Sub checkSaveDate()
If ActivePresentation.Path = "" Then Exit Sub ' not saved
MsgBox ActivePresentation.BuiltInDocumentProperties("Last Save Time")
MsgBox "The year only was " & Format(ActivePresentation.BuiltInDocumentProperties("Last Save Time"), "YYYY")
End Sub