ORA-01843: not a valid month

A date string has a month Oracle cannot accept for the format in use. Usually the day and month are swapped, the month name is in another language, or the session’s default date format differs. Always use TO_DATE with an explicit format.

What ORA-01843 means

When Oracle converts text to a date, it reads the text according to a format mask, or the session’s NLS_DATE_FORMAT if there is none. If the part in the month position is not a valid month, ORA-01843 follows. A query that works in one tool can fail in another simply because their NLS settings differ.

Common causes of ORA-01843

Day and month swapped, for example 15/10/2026 read as month 15.
A month name in another language: OCAK under English settings, JAN under Turkish ones.
Relying on an implicit conversion and the session’s NLS_DATE_FORMAT.
An invalid value in the data, such as month 13.

How to fix ORA-01843

Use an explicit mask: TO_DATE('2026-10-15', 'YYYY-MM-DD').
Or use an ANSI date literal, which has a fixed format: DATE '2026-10-15'.
For month names, set the language in the call: TO_DATE('15 OCAK 2026', 'DD MONTH YYYY', 'NLS_DATE_LANGUAGE=TURKISH').
Find bad rows: VALIDATE_CONVERSION(col AS DATE, 'YYYY-MM-DD') = 0 (Oracle 12.2 and later).

ORA-01843 example

There is no month 13:

-- ORA-01843: not a valid month
SELECT TO_DATE('2026-13-01', 'YYYY-MM-DD') FROM dual;

-- A date literal has no NLS dependency
SELECT DATE '2026-10-15' FROM dual;

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Oracle documentation for ORA-01843. Oracle is a registered trademark of Oracle Corporation. SQLlightspeed is not affiliated with Oracle.