2147483642::abstime is null, 2147483644::abstime is null

Hi,
why this statement:
SELECT  2147483647::abstime, 2147483646::abstime, 2147483645::abstime,
2147483644::abstime, 2147483643::abstime, 2147483642::abstime,
2147483641::abstime, 2147483640::abstime, 2147483639::abstime;

returns null form2147483646 and 2147483645? Is this feature or bug?
(Server version is 7.4.5).

Thanks.
Filip Jirsák