Oracle's default DATE format

A DATE has no format - it is stored internally as 7-bytes representing year (2 bytes) and month, day, hour, minute and second (1 byte each).

'25-JAN-18' is not a date - it is a text literal.

When you do:

INSERT INTO table_name ( date_column ) VALUES ( '25-JAN-18' );

Oracle will try to be helpful and perform an implicit cast from a string to a date using the NLS_DATE_FORMAT parameter for the user's session as the format model. So, your statement will be implicitly converted to:

INSERT INTO table_name ( date_column ) VALUES (
  TO_DATE(
    '25-JAN-18',
    ( SELECT VALUE FROM NLS_SESSION_PARAMETERS WHERE PARAMETER = 'NLS_DATE_FORMAT' )
  )
);

Any user can set their NLS parameters in their own session (so you should never rely on implicit conversion as each user can have a different settings for their own session and can change the values mid-session). Instead you should:

• Use a Date literal:

  DATE '2018-01-25'
  

• Use a Timestamp literal

  TIMESTAMP '2018-01-25 01:23:45'
  

• Use TO_DATE( date_string, format_string [, nls_values] ) and explicitly use a format model:

  TO_DATE( '25-JUN-18', 'DD-MON-RR' )
  

If you do want to change the NLS_DATE_FORMAT in your session then you can use:

ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';

What the default date format is?

Since a DATE does not have a format, this question does not make sense. Instead if we ask:

What is the default NLS_DATE_FORMAT session parameter that Oracle uses to convert between strings and dates?

It depends on the NLS_TERRITORY session parameter (so it depends where you are in the world):

SET SERVEROUTPUT ON;

VARIABLE cur REFCURSOR;

DECLARE
  territories SYS.ODCIVARCHAR2LIST;
  formats     SYS.ODCIVARCHAR2LIST := SYS.ODCIVARCHAR2LIST();
BEGIN
  select value
  BULK COLLECT INTO territories
  from v$nls_valid_values
  where parameter = 'TERRITORY'
  order by value;

  formats.EXTEND( territories.COUNT );
  FOR i IN 1 .. territories.COUNT LOOP
    EXECUTE IMMEDIATE 'ALTER SESSION SET NLS_TERRITORY='''||territories(i)||'''';

    SELECT value
    INTO   formats(i)
    FROM   NLS_SESSION_PARAMETERS
    WHERE  PARAMETER = 'NLS_DATE_FORMAT';
  END LOOP;

  OPEN :cur FOR
  SELECT CAST( f.format AS VARCHAR2(12) ) AS format,
         LISTAGG( t.territory, ', ' ) WITHIN GROUP ( ORDER BY t.territory ) AS territories
  FROM   ( SELECT ROWNUM AS rn, COLUMN_VALUE AS territory FROM TABLE( territories ) ) t
         INNER JOIN
         ( SELECT ROWNUM AS rn, COLUMN_VALUE AS format FROM TABLE( formats ) ) f
         ON ( f.rn = t.rn )
  GROUP BY f.format;
END;
/

PRINT :cur;

Outputs the date format and the list of territories corresponding to that format:

FORMAT       TERRITORIES
------------ ------------------------------------------------------------------
DD MON RRRR  THAILAND
DD-MM-RR     ALGERIA, BAHRAIN, INDIA, MOROCCO, THE NETHERLANDS, TUNISIA
DD-MM-RRRR   BANGLADESH, INDONESIA, ROMANIA, VIETNAM
DD-MON-RR    AMERICA, CHINA, HONG KONG, IRELAND, ITALY, PAKISTAN, TAIWAN,
             UNITED KINGDOM
DD-MON-RRRR  ISRAEL
DD.MM.RR     AUSTRIA, BELARUS, CIS, CROATIA, CZECH REPUBLIC, CZECHOSLOVAKIA,
             GERMANY, RUSSIA, SLOVAKIA, SLOVENIA, SWITZERLAND
DD.MM.RRRR   ALBANIA, AZERBAIJAN, ESTONIA, FINLAND, FYR MACEDONIA, ICELAND,
             KAZAKHSTAN, MACEDONIA, NORWAY, SERBIA AND MONTENEGRO, UKRAINE,
             YUGOSLAVIA
DD.MM.RRRR.  MONTENEGRO, SERBIA
DD.fmMM.RRRR ARMENIA
DD/MM/RR     AFGHANISTAN, BELGIUM, BRAZIL, CAMEROON, CATALONIA, CHILE, COLOMBIA,
             CONGO BRAZZAVILLE, CONGO KINSHASA, COSTA RICA, CYPRUS, DJIBOUTI,
             EGYPT, EL SALVADOR, FRANCE, GABON, GREECE, GUATEMALA, HONDURAS,
             IRAQ, IVORY COAST, JORDAN, KUWAIT, LEBANON, LIBYA, LUXEMBOURG,
             MAURITANIA, MEXICO, NEW ZEALAND, NICARAGUA, OMAN, PANAMA, PERU,
             PUERTO RICO, QATAR, SAUDI ARABIA, SINGAPORE, SOMALIA, SPAIN, SUDAN,
             SYRIA, UNITED ARAB EMIRATES, URUGUAY, VENEZUELA, YEMEN
DD/MM/RRRR   ARGENTINA, BAHAMAS, BERMUDA, ECUADOR, MALAYSIA, SENEGAL, TURKEY,
             UGANDA, ZAMBIA
DD/MON/RR    AUSTRALIA, SOUTH AFRICA, UZBEKISTAN
DD/fmMM/RRRR LAOS, NIGERIA
MM/DD/RRRR   PHILIPPINES
RR-MM-DD     CANADA, DENMARK, JAPAN
RR-MON-DD    HUNGARY
RR.MM.DD     PORTUGAL
RR/MM/DD     KOREA, POLAND
RRRR-MM-DD   BULGARIA, SWEDEN
RRRR-fmMM-DD CAMBODIA
RRRR.MM.DD   LATVIA, LITHUANIA
RRRR/fmMM/fm IRAN, SRI LANKA
fmDD-MM-RR   BOLIVIA
fmDD/MM/RR   PARAGUAY
fmDD/MM/RRRR BELIZE, ETHIOPIA, MALTA, NEPAL
fmDD/fmMM/RR MALDIVES
fmMM.DD.RRRR BOSNIA AND HERZEGOVINA
fmMM/DD/RRRR KENYA, TANZANIA

DATEs are DATEs - the format determines how the DATE will be displayed when you query your data.

If you don't supply a format and use the TO_CHAR function, we will provide the DATE back in the default NLS_DATE_FORMAT - which is defined in the database, but can also be specified for your session.

For your session -

select * from NLS_SESSION_PARAMETERS
where PARAMETER = 'NLS_DATE_FORMAT';

Mine is 'DD-MON-YYYY', so when I query SYSDATE:

SQL> select sysdate from dual;

SYSDATE    
-----------
03-MAY-2018

When working with DATEs, it's generally best practice NOT to assume a specific DATE format. So when both INSERT or SELECT data with DATEs, be EXPLICIT. For example.

SQL> drop table JUST_DATES;

Table JUST_DATES dropped.

SQL> 
SQL> create table JUST_DATES (
  2      DATE1 date
  3  );

Table JUST_DATES created.

SQL> 
SQL> insert   into JUST_DATES values ( to_date('01-01-2018','MM-DD-YYYY') );

1 row inserted.

SQL> 
SQL> select to_char(
  2      DATE1,
  3      'MON/DD/RR'
  4  )
  5    from JUST_DATES;

TO_CHAR(DATE1,'MON
------------------
JAN/01/18

Now, to answer your question - what's the default? Well, it depends.

It is derived from your NLS_TERRITORY (docs)

But, if you remember best practices, and you never assume what the default is, and you are explicit with your date formats when working with DATEs, you should be ok.

The best guide we have for this topic is our Globalization Support Guide. This is the section of interest to you.

Only thing you have to take care while inserting date using TO_DATE is how you parse it.

Example:
What will not work
to_date('31-03-2016','dd-mon-yyyy')

What will work
to_date('31-03-2016','dd-mm-yyyy') to_date('31-Mar-2016','dd-Mon-yyyy')

The date you are passing to to_date and format should match.
31 - dd
Mar - Mon
2016 - yyyy