now.sql

CREATE OR REPLACE FUNCTION public.NOW() RETURNS timestamp AS 'SELECT timestamp ''2016-01-01''' LANGUAGE SQL IMMUTABLE;

SELECT NOW();

BEGIN;
  SET LOCAL search_path TO public,pg_catalog;
  SELECT NOW();
COMMIT;

SELECT NOW();

DROP FUNCTION public.NOW();

result.txt

CREATE FUNCTION
              now              
-------------------------------
 2016-02-09 18:50:13.284655+02
(1 row)

BEGIN
SET
         now         
---------------------
 2016-01-01 00:00:00
(1 row)

COMMIT
              now              
-------------------------------
 2016-02-09 18:50:13.285317+02
(1 row)

DROP FUNCTION