quick Date question

select '12/13/1901 18:00:00 CST'::datetime;
?column?
----------------------------
Fri Dec 13 18:00:00 1901 CST
(1 row)

select '12/13/1901 17:59:59 CST'::datetime;
?column?
------------------------
Fri Dec 13 23:59:59 1901
(1 row)

Why???