=DLookUp("DateUpdate","MSysObjects","Name='NameOfForm'")
To avoid having to hard-code the form name, you can set the control source
in the form's Load event:
Me.txtModified.ControlSource =
"=DLookUp(""DateUpdate"",""MSysObjects"",""Name='" & Me.Name & "'"")"
I've only tested this in Access 97 (where each form does get a different
modified date). I would think that in Access 2000 and higher, all forms
would have the same modified date, due to the monolithic save.