SQL & PL/SQL

I am wanting to create a table that has a default value of a user defined function and because this default value is going to be used in a few different places hence I would like to use a function rather than in the table definition.

This is my current code which works fine:

CREATE TABLE DEPT_HIST
(
  DEPTNO      NUMBER(2) not null,
  DNAME       VARCHAR2(14),
  LOC         VARCHAR2(13),
  SQL_ACTV_C  CHAR(1) not null,
  EFFT_STRT_S TIMESTAMP(6) default SYSTIMESTAMP not null,
  EFFT_END_S  TIMESTAMP(6) default TO_TIMESTAMP('9999/12/30 00:00:00.000000', 'YYYY/MM/DD:HH24:MI:SS.FF6') not null,
  DELT_F      CHAR(1) default 'N' not null
);

but I would like to get something similar to this to work:

CREATE OR REPLACE FUNCTION EOT
   RETURN timestamp
IS
   Result   timestamp;
BEGIN
   RETURN (TO_TIMESTAMP ('9999/12/30 00:00:00.000000',
                         'YYYY/MM/DD:HH24:MI:SS.FF6'));
END EOT;
/

select eot from dual;

EOT
---------------------------------------------------------------------------
30/DEC/99 12:00:00.000000000 AM

CREATE TABLE DEPT_HIST
(
  DEPTNO      NUMBER(2) not null,
  DNAME       VARCHAR2(14),
  LOC         VARCHAR2(13),
  SQL_ACTV_C  CHAR(1) not null,
  EFFT_STRT_S TIMESTAMP(6) default SYSTIMESTAMP not null,
  EFFT_END_S  TIMESTAMP(6) default EOT not null,
  DELT_F      CHAR(1) default 'N' not null
);

but I get an error of:

  EFFT_END_S  TIMESTAMP(6) default EOT not null,
                                   *
ERROR at line 8:
ORA-00984: column not allowed here

Any ideas? I guess I could use a trigger but not exactly what I am after.