SQL> create table test (id number(1), datum date);

Table created.

SQL> insert all
  2    into test (id, datum) values (1, date '2012-12-31')
  3    into test (id, datum) values (2, date '2013-08-02')
  4  select * from dual;

2 rows created.

SQL> select id, to_char(datum, 'dd.mm.yyyy') dat
  2  from test;

        ID DAT
---------- ----------
         1 31.12.2012
         2 02.08.2013

SQL>

SQL> CREATE TABLE OT_REQQ(RH_NO NUMBER,RH_DT DATE);

Table created.

SQL> INSERT INTO OT_REQQ VALUES (1,'04-DEC-2012');

1 row created.

SQL> COMMIT;

Commit complete.

SQL> SELECT * FROM OT_REQQ;

     RH_NO RH_DT
---------- ---------
         1 04-DEC-12

--I changed the format trigger as below.

begin
 RPT2XLS.PUT_CELL(1,TO_CHAR(TO_DATE(:rh_dt),'DD/MM/RRRR') );
 RPT2XLS.NEW_LINE;
  return (TRUE);
end;