PostgreSQL: round time function

round.sql

CREATE OR REPLACE FUNCTION round_minutes( TIMESTAMP WITH TIME ZONE, integer)
RETURNS TIMESTAMP WITH TIME ZONE AS $$
  SELECT date_trunc('hour', $1) + (cast(($2::varchar||' min') as interval) * round( (date_part('minute',$1)::float + date_part('second',$1)/ 60.)::float / cast($2 as float)))
$$ LANGUAGE SQL IMMUTABLE;

CREATE OR REPLACE FUNCTION floor_minutes( TIMESTAMP WITH TIME ZONE, integer )
RETURNS TIMESTAMP WITH TIME ZONE AS $$
    SELECT round_minutes( $1 - cast((($2/2)::varchar ||' min') as interval ), $2 );
$$ LANGUAGE SQL IMMUTABLE;

CREATE OR REPLACE FUNCTION ceiling_minutes( TIMESTAMP WITH TIME ZONE, integer )
RETURNS TIMESTAMP WITH TIME ZONE AS $$
    SELECT round_minutes( $1 + cast((($2/2)::varchar ||' min') as interval ), $2 );
$$ LANGUAGE SQL IMMUTABLE;

CREATE OR REPLACE FUNCTION round_hours( TIMESTAMP WITH TIME ZONE, integer)
RETURNS TIMESTAMP WITH TIME ZONE AS $$
   SELECT date_trunc('day', $1) + (cast(($2::varchar||' hour') as interval) * round( (date_part('hour',$1)::float + date_part('minute',$1)/ 60. + date_part('second',$1)/ 3600.)::float / cast($2 as float)))
$$ LANGUAGE SQL IMMUTABLE;

CREATE OR REPLACE FUNCTION floor_hours( TIMESTAMP WITH TIME ZONE, integer )
RETURNS TIMESTAMP WITH TIME ZONE AS $$
    SELECT round_hours( $1 - cast((($2/2)::varchar ||' hour') as interval ), $2 );
$$ LANGUAGE SQL IMMUTABLE;

CREATE OR REPLACE FUNCTION ceiling_hours( TIMESTAMP WITH TIME ZONE, integer )
RETURNS TIMESTAMP WITH TIME ZONE AS $$
    SELECT round_hours( $1 + cast((($2/2)::varchar ||' hour') as interval ), $2 );
$$ LANGUAGE SQL IMMUTABLE;