Oracle 18c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
SELECT TRUNC(to_date('2012-07-18 13:27:18', 'YYYY-MM-DD HH24:MI:SS')) FROM DUAL;

TRUNC(TO_DATE('2012-07-1813:27:18','YYYY-MM-DDHH24:MI:SS'))

2012-07-18 00:00:00

ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD';
SELECT TRUNC(to_date('2012-07-18 13:27:18', 'YYYY-MM-DD HH24:MI:SS')) FROM DUAL;

TRUNC(TO_DATE('2012-07-1813:27:18','YYYY-MM-DDHH24:MI:SS'))

2012-07-18

SELECT TO_CHAR(
         TRUNC( to_date('2012-07-18 13:27:18', 'YYYY-MM-DD HH24:MI:SS') ),
         'DD/MM/YYYY'
       )
FROM   DUAL;

TO_CHAR(TRUNC(TO_DATE('2012-07-1813:27:18','YYYY-MM-DDHH24:MI:SS')),'DD/MM/YYYY')

18/07/2012