关于类型的一个小问题

CREATE OR REPLACE  function fnD2C
(ltm_value IN long)
return varchar2/////******为什么这不能用varchar(20)呢
as
  sztm_value varchar2(20) ;
begin
    select to_char(TO_DATE('1970/01/01:00:00:00', 'yyyy/mm/dd:hh24:mi:ss') + (ltm_value+ 28800)/86400, 'yyyy/mm/dd:hh24:mi:ss') INTO sztm_value from dual;

   return sztm_value;
end;