SQL> select to_date('01-JAN-13')+.99999 from dual;

 
TO_DATE('01-JAN-13')+.99999
---------------------------
2013-1-1 23:59:59
 
SQL> select to_date('01-JAN-13')+.999994 from dual;
 
TO_DATE('01-JAN-13')+.999994
----------------------------
2013-1-1 23:59:59
 
SQL> select to_date('01-JAN-13')+.999995 from dual;
 
TO_DATE('01-JAN-13')+.999995
----------------------------
2013-1-2
 
SQL> select to_date('01-JAN-13')+.999999 from dual;
 
TO_DATE('01-JAN-13')+.999999
----------------------------
2013-1-2