Don, sorry for not mentioning this -- it's just a copy paste from pgaccess which, IIRC, handles apostrophies when doing a save. Besides, if I put a dummy v_rz := now(); instead of v_rz := next_day__somedate + interval v_add_interval; it all works (and so does v_rz := next_day__somedate;). There is something wrong with interval part.
