TRUNC(<date_time>)
 
CREATE TABLE t (
datecol DATE);
 
INSERT INTO t (datecol) VALUES (SYSDATE);
 
INSERT INTO t (datecol) VALUES (TRUNC(SYSDATE));
 
INSERT INTO t (datecol) VALUES (TRUNC(SYSDATE, 'HH'));
 
INSERT INTO t (datecol) VALUES (TRUNC(SYSDATE, 'MI'));
 
COMMIT;
 
SELECT TO_CHAR(datecol, 'DD-MON-YYYY HH:MI:SS')
FROM t;
 
--------------------------------------------
 
TRUNC(<date_time>, '<format>')
 
SELECT TO_CHAR(SYSDATE, 'DD-MON-YYYY HH:MI:SS')
FROM dual;
 
-- first day of the month
SELECT TO_CHAR(TRUNC(SYSDATE, 'MM'), 'DD-MON-YYYY HH:MI:SS')
FROM dual;
 
SELECT TO_CHAR(TRUNC(SYSDATE, 'MON'), 'DD-MON-YYYY HH:MI:SS')
FROM dual;
 
SELECT TO_CHAR(TRUNC(SYSDATE, 'MONTH'), 'DD-MON-YYYY HH:MI:SS')
FROM dual;
 
-- first day of the year
SELECT TO_CHAR(TRUNC(SYSDATE, 'YYYY'), 'DD-MON-YYYY HH:MI:SS')
FROM dual;
 
SELECT TO_CHAR(TRUNC(SYSDATE, 'YEAR'), 'DD-MON-YYYY HH:MI:SS')
FROM dual;