Last day, last time

 

select last_day(add_months(sysdate,-1)) from dual;
select last_day(add_months(trunc(sysdate+1)- INTERVAL '1' SECOND,-1)) from dual;

SELECT TRUNC(SYSDATE + 1) - 1/(24*60*60) FROM DUAL
SELECT TRUNC(SYSDATE + 1) - INTERVAL '1' SECOND FROM DUAL;