select  to_date(mw_onset_date,'dd/mm/yyyy')as mw_onset_date,
        trunc(mws_last_update_date) as mws_last_update_date,
        trunc(mws_last_update_date)-to_date(mw_onset_date,'dd/mm/yyyy') as DaysBetween,
        pt_code
from
(
SELECT
    substr(mw.mw_onset_date,7,2)||'/'||substr(mw.mw_onset_date,5,2)||'/'||substr(mw.mw_onset_date,1,4) as mw_onset_date,
    trunc(mw.mws_last_update_date) as mws_last_update_date,
    c.pt_code
FROM
  K_MWREG mw, k_cpireg c  
WHERE
 c.pt_code = mw.pt_code
 and  mws_last_update_date is not null
 and substr(mw.mw_onset_date,5,2) <> 00
ORDER BY 
 mw.mw_onset_date desc
 )sub
 where mw_onset_date is not null
 and mw_onset_date > to_date('01/01/2009','dd/mm/2015')

  1  DECLARE
  2  STR1 VARCHAR2(8) := '20150508';
  3  DATE1 DATE := TO_DATE('01/05/2015','DD/MM/YYYY');
  4  DATE2 DATE;
  5  NUM1 NUMBER;
  6  BEGIN
  7  DATE2 :=  TO_DATE(STR1,'YYYYMMDD');
  8  DBMS_OUTPUT.PUT_LINE(DATE2);
  9  NUM1  := DATE2 - DATE1;
 10  DBMS_OUTPUT.PUT_LINE(NUM1 );
 11* END;
SQL> /
08-MAY-15
7

PL/SQL procedure successfully completed.

select to_date('20150508','yyyymmdd') - to_date('01/05/2015','dd/mm/yyyy') from dual;

SELECT
     c.pt_code,
     to_date(mw.mw_onset_date,'yyyymmdd')as mw_onset_date,
     trunc(mw.mws_last_update_date) as mws_last_update_date ,
     trunc(mw.mws_last_update_date)- to_date(mw.mw_onset_date,'yyyymmdd')as DaysBetween     
FROM
     K_MWREG mw , k_cpireg c  
WHERE
     c.pt_code = mw.pt_code
     and mw.mws_last_update_date is not null
     and  mw.mw_onset_date not like '%00'