what is the length of the Date?

hi,
I'm confused with the following query result, could anybody explain the reason, why vsize(sysdate) return 8, vsize(hiredate) return 7?
I know that vsize(expr) defined as "Returns the number of bytes in the internal representation of expr", but sysdate have one more byte than date type column defined in table.
select sysdate, vsize(sysdate) from dual;
sysdate vsize(sysdate)
22-AUG-02 8
select hiredate, vsize(hiredate) from emp;
sysdate vsize(sysdate)
22-AUG-02 7
[ August 22, 2002: Message edited by: Lipman Li ]

I know the reason already, sysdate is function, hiredate is a column.
cheers