You are talking about "number string". Is your variable "settdate" a Julian date ? Can you give us an example value ?
This works for me :
declare
a varchar2(100);
b date;
begin
a := '2450450';
b := to_date(a,'J');
dbms_output.put_line(to_char(b,'DD(MM/YYYY'));
end;
/
SQL>
01/01/1997
SQL>PL/SQL procedure successfully completed.