SQL> select trunc(sysdate) - to_date(trunc(sysdate, 'yyyy')) from dual;

                                           86

select 
case when ISSue_Dt is null then (AS_of_date - ACTL_MTY_DT )
     when ACTL_MTY_DT is null then (AS_of_date - ISSue_Dt )
     else ACTL_MTY_DT 
end

from <your_table)

yy(Date) -> TO_CHAR(Date, 'YYYY') or EXTRACT (year FROM Date)
mm(Date) -> TO_CHAR(Date, 'MM') or EXTRACT (month FROM Date)
dd(Date) -> TO_CHAR(Date, 'DD') or EXTRACT (day FROM Date)

SELECT (months_between( trunc(Dateb, 'MM'), trunc(Datea, 'MM') )*30 + extract(day FROM Dateb) - extract(day FROM Datea)) / 360
FROM (
  SELECT <get_the value from appropriate date using the definition in link using CASE> Datea, <dtto> Dateb
  FROM <your_table>
);