Help - newbie question on using date in SQL

Hi All,
I'm using oracle 8.1.7.0 on win2K.
I don't understand why the first sql statement doesn't return rows. The only difference between the two is the date of the second query is using 4 digits of year.

Please give me some light on this.

TIA.
-Andi-

SQL> select ename, hiredate from emp
  2 where hiredate between to_date('20-FEB-81','DD-MON-YY') AND to_date('20-MAY-81','DD-MON-YY');

no rows selected

SQL> ed
Wrote file afiedt.buf

  1 select ename, hiredate from emp
  2* where hiredate between to_date('20-FEB-1981','DD-MON-YYYY') AND to_date('20-MAY-1981','DD-MON-YYY')
SQL> / ENAME HIREDATE
---------- ---------

ALLEN      20-FEB-81
WARD       22-FEB-81
JONES      02-APR-81
BLAKE      01-MAY-81