Triggers and Functions

I would like to create a function/trigger which will update a timestamp on table
A when an UPDATE is executed against table B

I tried this:

create table tokens (
token_idx BIGSERIAL PRIMARY KEY,
token varchar(128) not null unique,
last_seen timestamp default now()
)

create table user_token (
user_idx integer not null references users(user_idx) on delete cascade on
update cascade,
token_idx bigint not null references tokens(token_idx) on delete cascade on
update cascade,
h_msgs integer default 0,
s_msgs integer default 0,
constraint pkey_user_token PRIMARY KEY (user_idx, token_idx)
)

CREATE FUNCTION update_token(bigint) RETURNS void AS '
UPDATE tokens SET last_seen = now()
WHERE token_idx = $1 and last_seen < now();
' LANGUAGE SQL

CREATE TRIGGER touch_token AFTER UPDATE ON user_token FOR EACH ROW
EXECUTE PROCEDURE update_token(token_idx)

But it doesn't seem to change the filed 'last_seen' on the tokens table...