CREATE OR REPLACE FUNCTION gentstz(seconds int) RETURNS timestamptz AS
$$
BEGIN
RETURN '2016-05-24 17:00:00+00'::timestamptz + seconds * INTERVAL '1 second';
END;
$$
LANGUAGE PLPGSQL IMMUTABLE;

/*
test=# SELECT gentstz(72408438);
        gentstz
------------------------
 2018-09-09 20:27:18+02
*/