http://www.databasejournal.com/img/dateADD.sql

----------------------------FUNCTION
DateADD--------------------------------------------
Create or replace  Function DateADD(vchDatePart  varchar , intOP   in   int,
dt date)
/***********************************************************************************
--GEO--C--  GAjoseph
--GEO--E--  Select DateADD('dd' , -10, sysdate) from dual
--GEO--E--  Select DateADD('dd' , 30, to_date('31-FEB-2003')) from dual
--GEO--E--  Select dateadd('m', 2 , sysdate), to_char(DateADD ( 'h' , 3 ,
to_date('01-JAN-2003 10:33:44' , 'dd-MON-YYYY HH:MI:SS') )  , 'dd-MON-YYYY
HH:MI:SS' )
           , DateADD ( 'h' , 3 , to_date('01-JAN-2003 10:33:44' ,
'dd-MON-YYYY HH:MI:SS') ) From dual;
--GEO--C--  Returns a new datetime value based on adding an interval to the
specified
           date.
--GEO--Ex--
           Is the parameter that specifies on which part of the date to
return a new value. The table lists the dateparts and abbreviations
recognized by Microsoft® SQL Server™.
           Datepart            Abbreviations
               Year                yy, yyyy
               Month               mm, m
               Day                 dd, d
               Hour                hh
               minute              mi, n
               second              ss, s

************************************************************************************/
return date
as
   dd      int;
   mm      int;
   yyyy    int;
   hh      int;
   NN      int;
   SS      int;
   v       date;
   lintOP  int;
Begin
   lintOP := intOP;
--GEO--C--  INcreament  Days
   if upper(vchDatePart) like 'D%'  then
       return dt + intOP;
   end if;
   dd  := to_number(to_Char(dt,'dd'));
   mm  := to_number(to_Char(dt,'MM'));
   yyyy:= to_number(to_Char(dt,'yyyy'));
   HH  := to_number(to_Char(dt, 'HH'));
   NN  := to_number(to_Char(dt, 'MI'));
   SS  := to_number(to_Char(dt, 'SS'));
--GEO--C--  INcreament  Year
   if upper(vchDatePart) like 'Y%' then
       yyyy:= yyyy+ lintOP;
   end if;
--GEO--C--  INcreament  Month.
   if upper(vchDatePart) like 'M%' then
       yyyy:= yyyy+round(lintOP/12);
       mm  := mm+mod(lintOP,12);
   end if;-->MM
   if upper(vchDatePart) like 'H%' then
       dd  :=  dd  +   round(lintOP/24);
       hh  :=  hh  +   mod(lintOP,24);
   end if;--> hh
   if upper(vchDatePart) like 'N%' then
       dd  :=  dd  +   round(lintOP/(24*60));
       hh  :=  hh  +   round(lintOP/60);
       NN  :=  NN  +   mod(lintOP , 60);
   end if;--> MInutes
   if upper(vchDatePart) like 'S%' then
       dd  :=  dd  +   round(lintOP/(24*60*60));
       hh  :=  hh  +   round(lintOP/60*60);
       NN  :=  NN  +   round(lintOP/60);
       NN  :=  NN  +   MOD(lintOP,60);
   end if;--> SS
       v   := LAST_DAY(to_date('01/'||to_char(mm,'09')||'/'|| to_char(yyyy,
'0009'),'dd/mm/yyyy'));
       if dd > to_number(to_Char(v,'DD'))  then
           dd := to_number(to_Char(v,'DD'));
       end if;
   return to_date(lpad(dd,2,'0')||to_char(mm,'09')||'/'|| to_char(yyyy,
'0009')||' '||lpad(hh,2,'0')||':'||lpad(NN,2,'0')||':'||lpad(SS,2,'0'),
'dd/mm/yyyy HH24:MI:SS') ;
   exception  when others then return null ;
End;