Oracle Datetime difference issue with AM/ PM - oracle

Question

I have the following stored procedure which calculates the time taken for a merge statement
create or replace procedure ModAuditData(
O_UpdatedCount out int
,O_EndTime out timestamp
,O_Duration out int)
as
P_StartTime timestamp(3) WITH LOCAL TIME ZONE;
-- EndTime timestamp;
begin
P_StartTime:=to_timestamp(to_char(current_timestamp,'DD/MM/YYYY HH:MI:SS'),'DD/MM/YYYY HH:MI:SS');
-- merge Statment that does UPSERT
O_UpdatedCount :=SQL%ROWCOUNT;
commit;
O_EndTime:=to_timestamp(to_char(current_timestamp,'DD/MM/YYYY HH:MI:SS'),'DD/MM/YYYY HH:MI:SS');
begin
select extract( second from (O_EndTime-P_StartTime) )
into O_Duration
from dual;
Exception When others then
O_Duration:=0;
end;
end ModAuditData;
The Issue is
O_EndTime:=to_timestamp(to_char(current_timestamp,'DD/MM/YYYY HH:MI:SS'),'DD/MM/YYYY HH:MI:SS')
gives exact opposite of
P_StartTime:=to_timestamp(to_char(current_timestamp,'DD/MM/YYYY HH:MI:SS'),'DD/MM/YYYY HH:MI:SS');
in terms of AM/PM
What is the correct way to calculate the start and end time

Answer 1

CURRENT_TIMESTAMP is already a TIMESTAMP WITH TIME ZONE, there is no reason to convert it first to VARCHAR2 and then back again into a TIMESTAMP.
Also timestamp(3) (which provides precision up to millisecond) does not make much sense when you return duration as INTEGER, i.e. full seconds.
Try it like this:
P_StartTime timestamp(3) WITH TIME ZONE;
begin
P_StartTime := current_timestamp;
-- merge Statment that does UPSERT
O_UpdatedCount :=SQL%ROWCOUNT;
commit;
O_Duration := EXTRACT(SECOND FROM (current_timestamp - P_StartTime));
end;
In case of SQL*Plus consider to use TIMING command.