SQL & PL/SQL

Dear experts ,

                 here is my function to calculate difference between two dates.

create or replace

Function  hour_diff

(p_p_StartDt TIMESTAMP,

p_p_EndDt TIMESTAMP)

Return Varchar2

As

v_TotalSec number(10);

v_DiffSecs number(10);

  v_m_DaysHour Varchar2(15);

Begin

--SELECT 24*60*60 INTO v_TotalSec FROM DUAL;

--SELECT (p_p_EndDt - p_p_StartDt) * 86400 INTO v_DiffSecs FROM DUAL;

  select days || 'd' || ' '||

hours || 'h' || ' '||

minutes || 'm' into v_m_DaysHour

from(

select extract( day from diff ) days,

           extract( hour from diff ) hours,

           extract( minute from diff ) minutes

           from (select to_timestamp( p_p_StartDt, 'yyyy-mm-dd' ) - to_timestamp( p_p_EndDt, 'yyyy-mm-dd' ) diff

             from dual));

Return    v_m_DaysHour;

End;

op required

---------

1d 2h 40m

tried function

select hour_diff('18-04-17 07:48:25','17-04-17 07:48:25') from dual;

ORA-01843: not a valid month

01843. 00000 -  "not a valid month"

*Cause:   

*Action:

PLEASE HELP