Rounding datetimes and timestamps in PostgreSQL

I’m working on a time billing application and needed a way to round punches to the nearest 15 minute increment from within the Postgres DB. The function below is the most elegant solution I could come up with. It could be easily modified to always round-up or round-down by replacing ROUND() with either CEIL() or FLOOR(), respectively.

CREATE OR REPLACE FUNCTION round_timestamp(
		 ts timestamptz
		,round_secs int
                ) RETURNS timestamptz AS $$
        DECLARE
                _mystamp timestamp;
                _round_secs decimal;
        BEGIN

	_round_secs := round_secs::decimal;

	_mystamp := timestamptz 'epoch' 
			+ ROUND((EXTRACT(EPOCH FROM ts))::int / _round_secs) * _round_secs
			* INTERVAL '1 second';

	RETURN _mystamp;

END; $$ LANGUAGE plpgsql IMMUTABLE;

SELECT * FROM round_timestamp('2010-03-04 11:39:11', 900);
    round_timestamp
------------------------
 2010-03-04 11:45:00-05
(1 row)