Impress - date field update on save

In Impress, I want to insert a date field that updates on save, so I can put
"Last changed "
to show the date the slideshow was last updated.

Apparently this feature is in Writer, but I can’t find it in Impress.

In the Insert field commands menu, there are Date (fixed) and Date (variable).
Imho Date (variable) refers to opening a file and not to saving it.

Thanks. Yes, the Date (variable) just displays the current date when the slide is being viewed. It does not get inserted as the date of the last save.

The irritating thing is that LO Writer seems to have this feature, but Impress does not.