Всегда считал что (x-"любое число")-(y-"любое число") = x-y

Принимаем:
x=epoch FROM now()
y=epoch FROM cast(now() AS timestamp

Выполняем запрос:
SELECT (extract(epoch FROM now() - cast('2014-08-23 15:06:00' AS timestamp)) -
extract(epoch FROM cast(now() AS timestamp) - cast('2014-08-23 15:06:00' AS timestamp))) union
select (extract(epoch FROM now())- extract(epoch FROM cast(now() AS timestamp)))

Получаем
-21600
3600

Где ошибка никак понять не могу...