Can't find the right generated column syntax

Hi list,

I've got this table:
    date_start    timestamptz,
    date_end    temstamptz

and I'd like to auto-generate a 3rd column with a tstzrange:
    date_start    timestamptz,
    date_end    temstamptz,
    duration    tstzrange GENERATED ALWAYS AS ( ? ) STORED

but I can't find the right syntax to do so :/ (is it even possible ?)

Jean-Yves