My =LastModified() formula/macro is not updating

Hello -

I used the macro below to enable the =LastModified() formula. When I modify and save the workbook, the cell containing =LastModified() does not update, nor does the cell update when I close and re-open the workbook.

I would like this cell to update on its own and reflect the last modified date of the workbook.

My Trust Center macro setting is set to "Disable all macros with notification", and calculation options are set to Auto.

Any ideas? The macro is below. Thanks!

Function LastModified() As Date

LastModified = ActiveWorkbook.BuiltinDocumentProperties("Last Save Time")

End Function