grant alter system to <username>;

conn sys / as sysdba
grant alter system to system;

conn system/<password>

create or replace procedure change_fixed_date(dt date) as
sqls1 varchar2(30):='dd-mon-yyyy hh24:mi:ss';
begin
execute immediate 'alter session set nls_date_format='''||sqls1 || '''';
execute immediate 'alter system set fixed_date='''||dt||'''';
end;
/

grant execute on change_fixed_date to a;

create user a identified by a;
grant connect to a;
grant execute on change_fixed_date to a;

conn a/a
exec change_fixed_date('01-Oct-2009 01:00:00')