How to store only time; not date and time?

Answer

 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;