Function gives wrong results

Question

I've calculated it myself and get different results.
FUNCTION calc_customer_profit (p_custid in number)
RETURN NUMBER
IS
v_days number default 0;
v_profit number default 0;
cursor c_profitvalues is
     select ren.customer_id
     ,               ren.rent_dt
     ,               ren.return_dt
     ,               ren.daily_rate
     from rentals ren
     where ren.customer_id = p_custid
     order by ren.customer_id;
BEGIN
     for r_profitvalues in c_profitvalues loop
          v_days := Days_Between(r_profitvalues.rent_dt, r_profitvalues.return_dt);
          v_profit := (v_days*r_profitvalues.daily_rate)+v_profit;
     end loop;
     return v_profit;
END calc_customer_profit;

Answer 1

How about this:
SQL> create or replace
  2  FUNCTION Days_Between (first_dt IN DATE, second_dt IN DATE := sysdate) RETURN NUMBER
  3  IS
  4  BEGIN
  5    RETURN TO_NUMBER(TO_CHAR(second_dt, 'J')) - TO_NUMBER(TO_CHAR(first_dt , 'J'));
  6  END;
  7  /
Function created.
SQL>
SQL> set null {null}
SQL>
SQL> select dt1, dt2, Days_Between(dt1, dt2)
  2  from (
  3  select to_date('24.12.2004', 'DD.MM.YYYY') dt1, to_date('01.01.2005', 'DD.MM.YYYY') dt2 from dual
  4  union all
  5  select to_date('01.01.2005', 'DD.MM.YYYY') dt1, to_date('08.01.2005', 'DD.MM.YYYY') dt2 from dual
  6  union all
  7  select to_date('01.01.2005', 'DD.MM.YYYY') dt1, to_date('01.01.2005', 'DD.MM.YYYY') dt2 from dual
  8  union all
  9  select to_date(NULL, 'DD.MM.YYYY') dt1, to_date('01.01.2005', 'DD.MM.YYYY') dt2 from dual
10  union all
11  select to_date('01.01.2005', 'DD.MM.YYYY') dt1, to_date(NULL, 'DD.MM.YYYY') dt2 from dual
12  );
DT1       DT2       DAYS_BETWEEN(DT1,DT2)
24-DEC-04 01-JAN-05                     8
01-JAN-05 08-JAN-05                     7
01-JAN-05 01-JAN-05                     0
{null}    01-JAN-05 {null}
01-JAN-05 {null}    {null}
SQL>