Check date format in Oracle
SQL> select value from v$NLS_PARAMETERS where parameter = 'NLS_DATE_FORMAT';
VALUE
--------------------
DD-MON-RR
SQL> select sysdate from dual;
SYSDATE
---------
29-MAR-21
Change Date format at Session level
SQL> ALTER SESSION SET NLS_DATE_FORMAT = 'DD-MON-YYYY HH24:MI';
Session altered.
SQL> select sysdate from dual;
SYSDATE
-----------------
29-MAR-2021 20:00
Note:
MM Numeric month (e.g., 07)
MON Abbreviated month name (e.g., JUL)
MONTH Full month name (e.g., JULY)
DD Day of month (e.g., 24)
DY Abbreviated name of day (e.g., FRI)
YYYY 4-digit year (e.g., 1998)
YY Last 2 digits of the year (e.g., 98)
RR Like YY, but the two digits are ``rounded'' to a year in the range 1950 to 2049. Thus, 06 is considered 2006 instead of 1906
AM (or PM) Meridian indicator
HH Hour of day (1-12)
HH24 Hour of day (0-23)
MI Minute (0-59)
SS Second (0-59)
Change data format with function TO_CHAR in Query
SQL> select to_char(sysdate, 'dd-mon-yyyy hh24:mi:ss') from dual;
TO_CHAR(SYSDATE,'DD-MON-YYYYHH24:MI:SS')
-----------------------------------------
29-mar-2021 20:02:04
SQL> select to_char(sysdate, 'dd-mon-yyyy') from dual;
TO_CHAR(SYSDATE,'DD-MON-YYYY')
-----------------------------------------
29-mar-2021
Change Data format with NLS_LANG environment variable
NLS_LANG is set to “JAPANESE_JAPAN.JA16SJIS” in regedit setting of Windows of client software.

SQL> select value from v$NLS_PARAMETERS where parameter = 'NLS_DATE_FORMAT';
VALUE
-----------------------------------------
RR-MM-DD
SQL> select to_char(sysdate, 'dd-mon-yyyy') from dual;
TO_CHAR(SYSDATE,'DD-MON-YYYY')
-----------------------------------------
29-3îÄ -2021
SQL> select sysdate from dual;
SYSDATE
--------
21-03-29