select '2014-04-14 12:17:55.772'::timestamp - interval '2 seconds';
Para mayor flexibilidad es posible multiplicar el intervalo
select '2014-04-14 12:17:55.772'::timestamp - 2 * interval '1 second';
Si desea truncar al segundo
select date_trunc(
'second',
'2014-04-14 12:17:55.772'::timestamp - interval '2 seconds'
);