Postgres (psql ?) rounds all odd second values to even seconds fo r timestamp(0) data type

Looks like postgres will "round" all odd second values to even seconds for
timestamp(0).
Is this a known behaviour, or it's a bug ? The data type docs say nothing
about this, searching the mail archive/Google returned no relevant result.
BTW, I'm using the 7.3b5 version.

cnagy=> select '1999-01-28 18:17:15'::timestamp(0);
timestamp
---------------------
1999-01-28 18:17:16
(1 row)

cnagy=> select '1999-01-28 00:00:01'::timestamp(0);
timestamp
---------------------
1999-01-28 00:00:02
(1 row)

The same happens for ANY date having the seconds an odd number.

Cheers,
Csaba.