select date_trunc('minute', now())

select date_trunc('minute', now() + interval '30 second')

CREATE FUNCTION round_minutes(TIMESTAMP WITHOUT TIME ZONE, integer)
RETURNS TIMESTAMP WITHOUT TIME ZONE AS $$
  SELECT date_trunc('hour', $1) + cast(($2::varchar||' min') as interval) * round(date_part('minute',$1)::float / cast($2 as float))
$$ LANGUAGE SQL;

SELECT round_minutes('2010-09-17 16:23:12', 5);
-- 2010-09-17 16:25:00