SQL & PL/SQL

I am not familiar with intervals and I misunderstand something.
I need to write a function that returns and interval comprising from current_timestamp to any other date:

Something like:

sql>select current_timestamp -(sysdate-100) from dual;

CURRENT_TIMESTAMP-(SYSDATE-100)
---------------------------------------------------------------------------
+000000100 00:00:00.152000

I have created a function:

CREATE OR REPLACE FUNCTION test_interval RETURN INTERVAL DAY TO SECOND IS
tmpVar INTERVAL DAY(0) TO SECOND(9);
BEGIN
   dbms_output.put_line('Init test_interval');
   --tmpVar := current_timestamp - (sysdate - 100);
   select current_timestamp - (sysdate - 100)
     into tmpVar
     from dual;
   dbms_output.put_line('End test_interval');
   RETURN tmpVar;
   EXCEPTION
     WHEN OTHERS THEN
       dbms_output.put_line(sqlerrm);
       raise;
END test_interval;
/

And when I try it I get the ORA-01873

sql>r
  1* select test_interval from dual
select test_interval from dual
       *
ERROR at line 1:
ORA-01873: the leading precision of the interval is too small
ORA-06512: at "FERNANDO.TEST_INTERVAL", line 14

I have tried changing the leading precision for tmpVar but the result is always the same.

Can someone tell me what I am misunderstanding?