Date arithmetic can be convoluted in Oracle. A customer wanted to know 
how to find all the records entered into a table since midnight. In 
order to do this, you can convert the column containing the timestamp 
to character with the format in decreasing chronological order; year, 
month, day, hour, minute, second. You can also truncate a date field to 
drop the time and leave just the date. Now find the sysdate and subtract
one day.  Any record created since midnight will be selected.

unix> date
Fri Sep  1 13:32:00 EDT 2000

unix> sqlplus user/password@oracle_sid

SQL> alter session set nls_date_format='YYYY/MM/DD HH24:MI:SS' ;

SQL> select UPDATE_TIMESTAMP from purchase_log where
SQL>    to_char (UPDATE_TIMESTAMP, 'YYYY/MM/DD HH24:MI:SS')  >
SQL>    (select to_char (trunc (sysdate) - 1, 'YYYY/MM/DD HH24:MI:SS') 
	from dual) ;

UPDATE_TIMESTAMP
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2000/09/01 00:12:51