SQL & PL/SQL :: ORA-01861 - Literal Does Not Match Format String

I'm trying to do data mining on a web log which recorded one day web access information from a busy web server. I imported the data into Oracle Data miner, and created a table (WEBLOG). The idea is to create a new field, i.e. session, for the users so that each session could be thought as a representative of a user-intent (aka topic). Now based on this, data mining models would be used to cluster(group) the users based on their similarity. The first step is to prepare the data which involves using SQL queries. So first, all I did was to create a function for date and time. This is the following code I used,

create or replace function ssndate(p_date in varchar2 default '03-01-18',
p_time in varchar2)
return number
$if dbms_db_version.ver_le_10 $then
deterministic
$elsif dbms_db_version.ver_le_11 $then
result_cache
$end
as
begin
return trunc((to_date(p_date||' '||p_time, 'dd-mm-yy hh24:mi:ss')
- to_date('01-01-90','dd-mm-yy')) * (86400/2400));
end ssndate;
/

The function ssndate compiled successfully.The next step I took was to create a view through the following query,

create or replace view WEBLOG_VIEWS
as
select (select ssndate(LOG_DATE, LOG_TIME) from dual) as "SESSION_DT",
C_IP,
CS_USER_AGENT,
(CS_URI_STEM||'?'||CS_URI_QUERY) as WEB_LINK
from WEBLOG;

This was successful as well. The problem is in the next step where I try to do data grouping.

create table FINAL_WEBLOG as
select SESSION_DT, C_IP, CS_USER_AGENT,
listagg(WEB_LINK, ' ')
within group(order by C_IP, CS_USER_AGENT) "WEB_LINKS"
from WEBLOG_VIEWS
group by C_IP, CS_USER_AGENT, SESSION_DT
order by SESSION_DT
[code]....

the to_date function should be fine. In the data that I possess, the date and time are in no format. Example: 30118 and 0:00:09 respectively.