SQL & PL/SQL

Hi,

i've created this function which takes a TIMESTAMP as a parameter and returns a truncated TIMESTAMP (truncated on 30 minutes).

I receive an error ORA-01843 Not a valid date. I haven't got a clue what's going wrong?

create or replace function trunc30 (p_timestamp timestamp) return timestamp
as 
    p_result    timestamp;
begin 
    SELECT 
      CASE WHEN (to_number(to_char(p_timestamp,'MI')) < 15 OR to_number(to_char(p_timestamp,'MI')) >= 45) THEN
        to_char(trunc(p_timestamp,'HH'),'HH24:MI') 
      ELSE 
        to_char(trunc(p_timestamp,'HH') + 30/(60*24),'HH24:MI') 
      END
    into p_result from dual; 
    return p_result; 
end;

I use this code to test the function:

DECLARE
  P_TIMESTAMP TIMESTAMP;
  v_Return TIMESTAMP;
BEGIN
  P_TIMESTAMP := TO_DATE('10/15/2008 15:44','MM/dd/yyyy HH24:MI');

  v_Return := TRUNC30(
    P_TIMESTAMP => P_TIMESTAMP
  );
  DBMS_OUTPUT.PUT_LINE('v_Return = ' || v_Return);
END;