SQL & PL/SQL :: Function To Find Max Last Logout Date For Each Agent Daily

I have to create a function. I need to find the max last logout date for each agent daily. For example, if an agent logged in for the first time at 9:00 and he logged out at 12:00 and he logged in again in 14:00 and he logged out at 15:00 the time I need my report to show is 15:00. How can I do that?In order to make it easiest for you to understand I am sending you this query:

select
a.login as login2,
To_Char(max(s.endtime), 'dd/MM/yyyy, HH24:MI:SS') as lastLogout
from cti.agent a
inner join cti.agentsessionlog s
on s.agentid = a.agentid and To_Char(s.endtime) != '31-DEC-99 11.59.59.000000 PM'
group by a.login;

This query returns the agent's login and the agent's last logout time. It works fine if I enter a date between but I cannot do that. If a use this query as it is and I try to export a report for 31/5 it shows as lastlogout the logout for 01/06 or 2/06. Is there a function I can use? I have a deadline.