can't create index with 'dowcast' row

Hi,

To constraint unique'ness of my visitors to a 24h periode I tried
created a index including the 'date' part of the created_on timestamp:

CREATE UNIQUE INDEX visit_idx ON visit_buffer USING btree (id_session,
id_story, created_on::date);

psql:visit_pkey.sql:5: ERROR: syntax error at or near "::"
LINE 1: ...buffer USING btree (id_session, id_story, created_on::date);

and this:

CREATE UNIQUE INDEX visit_idx ON visit_buffer USING btree (id_session, id_story, extract(date from created_on));
psql:visit_pkey.sql:4: ERROR: functions in index expression must be marked IMMUTABLE

How can I achieve what I am trying?

Thanks,