You could try the INTERVAL DAY TO SECOND data type but it won’t save you any disk space … it is very suitable for this purpose though.
create table t1 (time_of_day interval day (0) to second(0));
insert into t1 values (TO_DSINTERVAL('0 23:59:59'));
select date '2009-05-13'+time_of_day
from t1;
11 bytes though.