datetime fields have '60' in seconds field

When a DATETIME field is modified and the seconds are set to zero, the field
always shows 60 seconds instead:

testdb=> create table datetab (dt datetime);
CREATE
testdb=> insert into datetab (dt) values ('1999-12-31 12:05:00');
INSERT 27361 1
testdb=> select * from datetab;
dt
--------------------------
31/12/1999 12:05:60.00 MST
(1 row)

I am using Linux Mandrake 6.1, Postgres 6.5.1-7mdk, glibc 2.1.1-16mdk, kernel
2.2.13-7mdk