ORA-01843 Not a Valid Month - Fix Oracle Month Format Errors

ORA-01843: Not a Valid Month

Error Overview

Error Text: ORA-01843: not a valid month

This error occurs when Oracle cannot recognize the month portion of a date string during conversion. Common causes include invalid month values (like 13), incorrect month names, language/locale mismatches, or wrong date format masks.

Understanding Month Formats

Oracle Month Format Elements

Month Format Options
├── MM - Numeric month (01-12)
├── MON - Abbreviated month name (JAN, FEB)
├── MONTH - Full month name (JANUARY, FEBRUARY)
├── RM - Roman numeral month (I-XII)
└── Language-specific variations

Common Scenarios

• Invalid month numbers - Values outside 01-12 range
• Wrong month names - Typos or incorrect abbreviations
• Language mismatches - English months with non-English database
• Format confusion - DD/MM vs MM/DD formats
• Case sensitivity - Month names in wrong case

Diagnostic Steps

1. Identify Invalid Month Values

-- Check for invalid numeric months
SELECT
  date_string,
  SUBSTR(date_string, 6, 2) as month_part,
  CASE
    WHEN TO_NUMBER(SUBSTR(date_string, 6, 2)) BETWEEN 1 AND 12 THEN 'Valid'
    ELSE 'Invalid month: ' || SUBSTR(date_string, 6, 2)
  END as month_check
FROM import_table
WHERE REGEXP_LIKE(date_string, '^\d{4}-\d{2}-\d{2}')
  AND (TO_NUMBER(SUBSTR(date_string, 6, 2)) < 1
       OR TO_NUMBER(SUBSTR(date_string, 6, 2)) > 12);

-- Check current NLS settings
SELECT
  parameter,
  value
FROM nls_session_parameters
WHERE parameter IN ('NLS_DATE_FORMAT', 'NLS_DATE_LANGUAGE', 'NLS_TERRITORY');

-- Test month name recognition
DECLARE
  v_date DATE;
  v_months SYS.ODCIVARCHAR2LIST := SYS.ODCIVARCHAR2LIST(
    'JAN', 'FEB', 'MAR', 'APR', 'MAY', 'JUN',
    'JUL', 'AUG', 'SEP', 'OCT', 'NOV', 'DEC'
  );
BEGIN
  FOR i IN 1..v_months.COUNT LOOP
    BEGIN
      v_date := TO_DATE('15-' || v_months(i) || '-2024', 'DD-MON-YYYY');
      DBMS_OUTPUT.PUT_LINE(v_months(i) || ' - Valid');
    EXCEPTION
      WHEN OTHERS THEN
        DBMS_OUTPUT.PUT_LINE(v_months(i) || ' - Error: ' || SQLERRM);
    END;
  END LOOP;
END;
/

2. Analyze Date String Patterns

-- Detect month format in strings
CREATE OR REPLACE FUNCTION detect_month_format(
  p_date_string VARCHAR2
) RETURN VARCHAR2 AS
  v_month_part VARCHAR2(20);
BEGIN
  -- Check for numeric month (MM)
  IF REGEXP_LIKE(p_date_string, '\d{2}[-/]\d{2}[-/]\d{4}') THEN
    v_month_part := REGEXP_SUBSTR(p_date_string, '\d{2}', 1, 2);
    IF TO_NUMBER(v_month_part) BETWEEN 1 AND 12 THEN
      RETURN 'Numeric (MM): ' || v_month_part;
    ELSE
      RETURN 'Invalid numeric month: ' || v_month_part;
    END IF;

  -- Check for abbreviated month (MON)
  ELSIF REGEXP_LIKE(p_date_string, '[A-Z]{3}', 'i') THEN
    v_month_part := REGEXP_SUBSTR(p_date_string, '[A-Z]{3}', 1, 1, 'i');
    RETURN 'Abbreviated (MON): ' || v_month_part;

  -- Check for full month name (MONTH)
  ELSIF REGEXP_LIKE(p_date_string, '[A-Z]{4,}', 'i') THEN
    v_month_part := REGEXP_SUBSTR(p_date_string, '[A-Z]{4,}', 1, 1, 'i');
    RETURN 'Full name (MONTH): ' || v_month_part;
  ELSE
    RETURN 'Unknown format';
  END IF;
END;
/

-- Test with various formats
SELECT
  date_string,
  detect_month_format(date_string) as month_format
FROM (
  SELECT '15-01-2024' as date_string FROM dual UNION ALL
  SELECT '15-JAN-2024' FROM dual UNION ALL
  SELECT '15-JANUARY-2024' FROM dual UNION ALL
  SELECT '15-13-2024' FROM dual UNION ALL
  SELECT '15-ABC-2024' FROM dual
);

3. Check Locale and Language Issues

-- Test month names in different languages
DECLARE
  v_date DATE;
  v_languages SYS.ODCIVARCHAR2LIST := SYS.ODCIVARCHAR2LIST(
    'AMERICAN', 'FRENCH', 'SPANISH', 'GERMAN', 'ITALIAN'
  );
BEGIN
  FOR i IN 1..v_languages.COUNT LOOP
    BEGIN
      EXECUTE IMMEDIATE 'ALTER SESSION SET NLS_DATE_LANGUAGE = ' || v_languages(i);
      v_date := TO_DATE('15-JAN-2024', 'DD-MON-YYYY');
      DBMS_OUTPUT.PUT_LINE(v_languages(i) || ': Success with JAN');
    EXCEPTION
      WHEN OTHERS THEN
        DBMS_OUTPUT.PUT_LINE(v_languages(i) || ': Failed - ' || SQLERRM);
    END;
  END LOOP;

  -- Reset to default
  EXECUTE IMMEDIATE 'ALTER SESSION SET NLS_DATE_LANGUAGE = AMERICAN';
END;
/

-- Find problematic month names
SELECT DISTINCT
  date_string,
  REGEXP_SUBSTR(date_string, '[A-Z]+', 1, 1, 'i') as month_text,
  CASE
    WHEN UPPER(REGEXP_SUBSTR(date_string, '[A-Z]+', 1, 1, 'i'))
         IN ('JAN','FEB','MAR','APR','MAY','JUN','JUL','AUG','SEP','OCT','NOV','DEC')
    THEN 'Valid English month'
    WHEN UPPER(REGEXP_SUBSTR(date_string, '[A-Z]+', 1, 1, 'i'))
         IN ('JANUARY','FEBRUARY','MARCH','APRIL','MAY','JUNE',
             'JULY','AUGUST','SEPTEMBER','OCTOBER','NOVEMBER','DECEMBER')
    THEN 'Valid full English month'
    ELSE 'Invalid or non-English month'
  END as month_validation
FROM import_table
WHERE REGEXP_LIKE(date_string, '[A-Z]+', 'i');

Immediate Solutions

Solution 1: Fix Month Values

Correct Invalid Month Numbers

-- Fix out-of-range month values
UPDATE import_table
SET date_string = REGEXP_REPLACE(date_string, '-13-', '-12-')
WHERE REGEXP_LIKE(date_string, '-13-');

-- Create function to validate and fix months
CREATE OR REPLACE FUNCTION fix_month_value(
  p_date_string VARCHAR2
) RETURN VARCHAR2 AS
  v_year VARCHAR2(4);
  v_month VARCHAR2(2);
  v_day VARCHAR2(2);
  v_fixed_month NUMBER;
BEGIN
  -- Extract parts (assuming YYYY-MM-DD format)
  v_year := SUBSTR(p_date_string, 1, 4);
  v_month := SUBSTR(p_date_string, 6, 2);
  v_day := SUBSTR(p_date_string, 9, 2);

  -- Fix month if invalid
  v_fixed_month := TO_NUMBER(v_month);

  IF v_fixed_month < 1 THEN
    v_fixed_month := 1;
  ELSIF v_fixed_month > 12 THEN
    v_fixed_month := 12;
  END IF;

  RETURN v_year || '-' || LPAD(v_fixed_month, 2, '0') || '-' || v_day;
EXCEPTION
  WHEN OTHERS THEN
    RETURN p_date_string;  -- Return original if can't fix
END;
/

-- Apply fixes
UPDATE import_table
SET date_string = fix_month_value(date_string)
WHERE TO_NUMBER(SUBSTR(date_string, 6, 2)) NOT BETWEEN 1 AND 12;

Handle Month Name Issues

-- Fix common month name errors
CREATE OR REPLACE FUNCTION fix_month_name(
  p_date_string VARCHAR2
) RETURN VARCHAR2 AS
  v_result VARCHAR2(100);
  TYPE month_map_type IS TABLE OF VARCHAR2(10) INDEX BY VARCHAR2(20);
  v_month_map month_map_type;
BEGIN
  -- Build correction map
  v_month_map('JANVIER') := 'JAN';
  v_month_map('FEVRIER') := 'FEB';
  v_month_map('MARS') := 'MAR';
  v_month_map('AVRIL') := 'APR';
  v_month_map('MAI') := 'MAY';
  v_month_map('JUIN') := 'JUN';
  v_month_map('JUILLET') := 'JUL';
  v_month_map('AOUT') := 'AUG';
  v_month_map('SEPTEMBRE') := 'SEP';
  v_month_map('OCTOBRE') := 'OCT';
  v_month_map('NOVEMBRE') := 'NOV';
  v_month_map('DECEMBRE') := 'DEC';

  v_result := UPPER(p_date_string);

  -- Replace foreign month names with English
  FOR month_name IN v_month_map.FIRST..v_month_map.LAST LOOP
    IF v_month_map.EXISTS(month_name) THEN
      v_result := REPLACE(v_result, month_name, v_month_map(month_name));
    END IF;
  END LOOP;

  RETURN v_result;
END;
/

-- Convert with language specification
SELECT TO_DATE('15-JANVIER-2024', 'DD-MONTH-YYYY', 'NLS_DATE_LANGUAGE=FRENCH') as french_date
FROM dual;

Solution 2: Handle Format Ambiguity

Detect and Convert DD/MM vs MM/DD

-- Smart date parser for ambiguous formats
CREATE OR REPLACE FUNCTION parse_ambiguous_date(
  p_date_string VARCHAR2,
  p_default_format VARCHAR2 DEFAULT 'DD/MM/YYYY'
) RETURN DATE AS
  v_parts APEX_T_VARCHAR2;
  v_part1 NUMBER;
  v_part2 NUMBER;
  v_year NUMBER;
  v_month NUMBER;
  v_day NUMBER;
BEGIN
  -- Split date string
  v_parts := APEX_STRING.SPLIT(p_date_string, '/');

  IF v_parts.COUNT != 3 THEN
    -- Try with dash separator
    v_parts := APEX_STRING.SPLIT(p_date_string, '-');
  END IF;

  IF v_parts.COUNT = 3 THEN
    v_part1 := TO_NUMBER(v_parts(1));
    v_part2 := TO_NUMBER(v_parts(2));
    v_year := TO_NUMBER(v_parts(3));

    -- Determine if DD/MM or MM/DD
    IF v_part1 > 12 AND v_part2 <= 12 THEN
      -- First part > 12, must be day
      v_day := v_part1;
      v_month := v_part2;
    ELSIF v_part2 > 12 AND v_part1 <= 12 THEN
      -- Second part > 12, must be day
      v_month := v_part1;
      v_day := v_part2;
    ELSE
      -- Both could be valid, use default format
      IF p_default_format LIKE 'DD%' THEN
        v_day := v_part1;
        v_month := v_part2;
      ELSE
        v_month := v_part1;
        v_day := v_part2;
      END IF;
    END IF;

    -- Validate and return
    IF v_month BETWEEN 1 AND 12 AND v_day BETWEEN 1 AND 31 THEN
      RETURN TO_DATE(v_year || '-' || v_month || '-' || v_day, 'YYYY-MM-DD');
    END IF;
  END IF;

  RAISE_APPLICATION_ERROR(-20001, 'Cannot parse date: ' || p_date_string);
END;
/

-- Test ambiguous dates
SELECT
  date_string,
  parse_ambiguous_date(date_string, 'DD/MM/YYYY') as dd_mm_result,
  parse_ambiguous_date(date_string, 'MM/DD/YYYY') as mm_dd_result
FROM (
  SELECT '13/01/2024' as date_string FROM dual UNION ALL  -- Unambiguous: day=13
  SELECT '01/13/2024' FROM dual UNION ALL  -- Unambiguous: month=13 (invalid)
  SELECT '05/06/2024' FROM dual  -- Ambiguous
);

Solution 3: Use NLS Settings

Set Correct Language and Territory

-- Set session NLS parameters
ALTER SESSION SET NLS_DATE_LANGUAGE = 'AMERICAN';
ALTER SESSION SET NLS_DATE_FORMAT = 'DD-MON-YYYY';

-- Convert with explicit NLS settings
SELECT
  TO_DATE('15-JAN-2024', 'DD-MON-YYYY', 'NLS_DATE_LANGUAGE=AMERICAN') as english_date,
  TO_DATE('15-ENE-2024', 'DD-MON-YYYY', 'NLS_DATE_LANGUAGE=SPANISH') as spanish_date,
  TO_DATE('15-JAN-2024', 'DD-MON-YYYY', 'NLS_DATE_LANGUAGE=FRENCH') as french_date
FROM dual;

-- Create conversion function with language detection
CREATE OR REPLACE FUNCTION convert_multilingual_date(
  p_date_string VARCHAR2
) RETURN DATE AS
  v_date DATE;
  TYPE lang_array IS TABLE OF VARCHAR2(30);
  v_languages lang_array := lang_array(
    'AMERICAN', 'SPANISH', 'FRENCH', 'GERMAN', 'ITALIAN', 'PORTUGUESE'
  );
BEGIN
  -- Try each language
  FOR i IN 1..v_languages.COUNT LOOP
    BEGIN
      v_date := TO_DATE(p_date_string, 'DD-MON-YYYY',
                       'NLS_DATE_LANGUAGE=' || v_languages(i));
      RETURN v_date;
    EXCEPTION
      WHEN OTHERS THEN
        NULL;  -- Try next language
    END;
  END LOOP;

  -- If no language worked, raise error
  RAISE_APPLICATION_ERROR(-20001,
    'Cannot convert date in any known language: ' || p_date_string);
END;
/

Long-Term Solutions

1. Implement Robust Date Parsing

Create Comprehensive Date Parser

CREATE OR REPLACE PACKAGE date_parser AS

  -- Month name mappings
  TYPE month_map_type IS TABLE OF VARCHAR2(10) INDEX BY VARCHAR2(30);

  FUNCTION parse_date(
    p_date_string VARCHAR2,
    p_format_hint VARCHAR2 DEFAULT NULL
  ) RETURN DATE;

  FUNCTION normalize_month(p_month_string VARCHAR2) RETURN VARCHAR2;

  FUNCTION validate_month(p_month VARCHAR2) RETURN BOOLEAN;

  PROCEDURE register_month_alias(
    p_alias VARCHAR2,
    p_standard VARCHAR2
  );

END date_parser;
/

CREATE OR REPLACE PACKAGE BODY date_parser AS

  g_month_aliases month_map_type;

  -- Initialize month aliases
  PROCEDURE init_month_aliases AS
  BEGIN
    -- English variations
    g_month_aliases('JANUARY') := '01';
    g_month_aliases('FEBRUARY') := '02';
    g_month_aliases('MARCH') := '03';
    g_month_aliases('APRIL') := '04';
    g_month_aliases('MAY') := '05';
    g_month_aliases('JUNE') := '06';
    g_month_aliases('JULY') := '07';
    g_month_aliases('AUGUST') := '08';
    g_month_aliases('SEPTEMBER') := '09';
    g_month_aliases('OCTOBER') := '10';
    g_month_aliases('NOVEMBER') := '11';
    g_month_aliases('DECEMBER') := '12';

    -- Abbreviations
    g_month_aliases('JAN') := '01';
    g_month_aliases('FEB') := '02';
    g_month_aliases('MAR') := '03';
    g_month_aliases('APR') := '04';
    g_month_aliases('JUN') := '06';
    g_month_aliases('JUL') := '07';
    g_month_aliases('AUG') := '08';
    g_month_aliases('SEP') := '09';
    g_month_aliases('OCT') := '10';
    g_month_aliases('NOV') := '11';
    g_month_aliases('DEC') := '12';

    -- Spanish
    g_month_aliases('ENERO') := '01';
    g_month_aliases('FEBRERO') := '02';
    g_month_aliases('MARZO') := '03';
    g_month_aliases('ABRIL') := '04';
    g_month_aliases('MAYO') := '05';
    g_month_aliases('JUNIO') := '06';
    g_month_aliases('JULIO') := '07';
    g_month_aliases('AGOSTO') := '08';
    g_month_aliases('SEPTIEMBRE') := '09';
    g_month_aliases('OCTUBRE') := '10';
    g_month_aliases('NOVIEMBRE') := '11';
    g_month_aliases('DICIEMBRE') := '12';
  END init_month_aliases;

  FUNCTION normalize_month(p_month_string VARCHAR2) RETURN VARCHAR2 AS
    v_upper_month VARCHAR2(30);
  BEGIN
    v_upper_month := UPPER(TRIM(p_month_string));

    -- Check if it's already numeric
    IF REGEXP_LIKE(v_upper_month, '^\d{1,2}$') THEN
      RETURN LPAD(v_upper_month, 2, '0');
    END IF;

    -- Look up in aliases
    IF g_month_aliases.COUNT = 0 THEN
      init_month_aliases;
    END IF;

    IF g_month_aliases.EXISTS(v_upper_month) THEN
      RETURN g_month_aliases(v_upper_month);
    END IF;

    -- Return as-is if not found
    RETURN p_month_string;
  END normalize_month;

  FUNCTION validate_month(p_month VARCHAR2) RETURN BOOLEAN AS
    v_month_num NUMBER;
  BEGIN
    v_month_num := TO_NUMBER(normalize_month(p_month));
    RETURN v_month_num BETWEEN 1 AND 12;
  EXCEPTION
    WHEN OTHERS THEN
      RETURN FALSE;
  END validate_month;

  FUNCTION parse_date(
    p_date_string VARCHAR2,
    p_format_hint VARCHAR2 DEFAULT NULL
  ) RETURN DATE AS
    v_normalized VARCHAR2(100);
    v_parts APEX_T_VARCHAR2;
    v_year VARCHAR2(4);
    v_month VARCHAR2(2);
    v_day VARCHAR2(2);
  BEGIN
    -- Normalize the date string
    v_normalized := TRIM(p_date_string);

    -- Try to extract parts
    IF REGEXP_LIKE(v_normalized, '\d{4}') THEN
      v_year := REGEXP_SUBSTR(v_normalized, '\d{4}');
    END IF;

    -- Extract month (could be text or number)
    IF REGEXP_LIKE(v_normalized, '[A-Z]+', 'i') THEN
      v_month := normalize_month(REGEXP_SUBSTR(v_normalized, '[A-Z]+', 1, 1, 'i'));
    ELSIF REGEXP_LIKE(v_normalized, '\d{1,2}[-/]\d{1,2}') THEN
      v_parts := APEX_STRING.SPLIT(REGEXP_SUBSTR(v_normalized, '\d{1,2}[-/]\d{1,2}'), '[-/]');
      -- Determine which is month based on value
      IF TO_NUMBER(v_parts(1)) > 12 THEN
        v_month := LPAD(v_parts(2), 2, '0');
        v_day := LPAD(v_parts(1), 2, '0');
      ELSE
        v_month := LPAD(v_parts(1), 2, '0');
        v_day := LPAD(v_parts(2), 2, '0');
      END IF;
    END IF;

    -- Build and return date
    IF v_year IS NOT NULL AND v_month IS NOT NULL AND v_day IS NOT NULL THEN
      RETURN TO_DATE(v_year || '-' || v_month || '-' || v_day, 'YYYY-MM-DD');
    END IF;

    RAISE_APPLICATION_ERROR(-20001, 'Cannot parse date: ' || p_date_string);
  END parse_date;

  PROCEDURE register_month_alias(
    p_alias VARCHAR2,
    p_standard VARCHAR2
  ) AS
  BEGIN
    g_month_aliases(UPPER(p_alias)) := p_standard;
  END register_month_alias;

BEGIN
  -- Initialize on package load
  init_month_aliases;
END date_parser;
/

2. Data Quality Validation

Create Validation Framework

-- Create month validation rules
CREATE TABLE month_validation_rules (
  rule_id NUMBER GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
  rule_name VARCHAR2(100),
  pattern VARCHAR2(200),
  valid_values CLOB,
  error_message VARCHAR2(500),
  active CHAR(1) DEFAULT 'Y'
);

-- Insert validation rules
INSERT INTO month_validation_rules (rule_name, pattern, valid_values, error_message)
VALUES ('Numeric Month', '^\d{2}$', '01,02,03,04,05,06,07,08,09,10,11,12',
        'Month must be between 01 and 12');

INSERT INTO month_validation_rules (rule_name, pattern, valid_values, error_message)
VALUES ('English Abbrev', '^[A-Z]{3}$', 'JAN,FEB,MAR,APR,MAY,JUN,JUL,AUG,SEP,OCT,NOV,DEC',
        'Invalid month abbreviation');

-- Validation procedure
CREATE OR REPLACE PROCEDURE validate_date_months(
  p_table_name VARCHAR2,
  p_column_name VARCHAR2
) AS
  v_invalid_count NUMBER;
  v_sql VARCHAR2(4000);
BEGIN
  -- Check for invalid numeric months
  v_sql := 'SELECT COUNT(*) FROM ' || p_table_name ||
           ' WHERE REGEXP_LIKE(' || p_column_name || ', ''\d{4}-\d{2}-\d{2}'')' ||
           ' AND TO_NUMBER(SUBSTR(' || p_column_name || ', 6, 2)) NOT BETWEEN 1 AND 12';

  EXECUTE IMMEDIATE v_sql INTO v_invalid_count;

  IF v_invalid_count > 0 THEN
    DBMS_OUTPUT.PUT_LINE('Found ' || v_invalid_count || ' records with invalid months');

    -- Log details
    INSERT INTO data_quality_issues (
      table_name, column_name, issue_type,
      record_count, discovered_date
    ) VALUES (
      p_table_name, p_column_name, 'Invalid Month',
      v_invalid_count, SYSDATE
    );
  END IF;
END;
/

Prevention Strategies

1. Input Validation

-- Add check constraint for valid months
ALTER TABLE date_table
ADD CONSTRAINT chk_valid_month
CHECK (
  TO_NUMBER(SUBSTR(date_column, 6, 2)) BETWEEN 1 AND 12
);

-- Create validation trigger
CREATE OR REPLACE TRIGGER validate_month_before_insert
BEFORE INSERT OR UPDATE ON import_table
FOR EACH ROW
DECLARE
  v_month NUMBER;
BEGIN
  IF :NEW.date_string IS NOT NULL THEN
    -- Extract and validate month
    v_month := TO_NUMBER(SUBSTR(:NEW.date_string, 6, 2));

    IF v_month NOT BETWEEN 1 AND 12 THEN
      RAISE_APPLICATION_ERROR(-20001,
        'Invalid month value: ' || v_month || '. Must be between 1 and 12.');
    END IF;
  END IF;
END;
/

2. Standardization Functions

-- Create function to standardize date input
CREATE OR REPLACE FUNCTION standardize_date_input(
  p_date_string VARCHAR2,
  p_expected_format VARCHAR2 DEFAULT 'YYYY-MM-DD'
) RETURN VARCHAR2 AS
  v_result VARCHAR2(30);
BEGIN
  -- Remove common issues
  v_result := TRIM(UPPER(p_date_string));

  -- Fix common month errors
  v_result := REPLACE(v_result, 'JANAURY', 'JANUARY');
  v_result := REPLACE(v_result, 'FEBUARY', 'FEBRUARY');

  -- Convert to standard format
  IF p_expected_format = 'YYYY-MM-DD' THEN
    -- Ensure numeric month
    v_result := REGEXP_REPLACE(v_result, 'JAN', '01');
    v_result := REGEXP_REPLACE(v_result, 'FEB', '02');
    -- ... etc for all months
  END IF;

  RETURN v_result;
END;
/

Related Errors

• ORA-01830 - Date format picture ends before converting
• ORA-01847 - Day of month must be between 1 and last day
• ORA-01858 - Non-numeric character where numeric expected
• ORA-01861 - Literal does not match format string

Quick Reference

Emergency Response Steps

1. ✓ Check month values are between 1-12
2. ✓ Verify month name spelling and case
3. ✓ Check NLS_DATE_LANGUAGE setting
4. ✓ Confirm date format (DD/MM vs MM/DD)
5. ✓ Fix or standardize month values
6. ✓ Use appropriate format mask

Quick Commands

-- Check for invalid months
SELECT * FROM table WHERE TO_NUMBER(SUBSTR(date_col, 6, 2)) NOT BETWEEN 1 AND 12;

-- Convert with language
SELECT TO_DATE('15-JAN-2024', 'DD-MON-YYYY', 'NLS_DATE_LANGUAGE=AMERICAN') FROM dual;

-- Fix month value
UPDATE table SET date_col = REPLACE(date_col, '-13-', '-12-') WHERE date_col LIKE '%-13-%';

-- Check NLS settings
SELECT * FROM nls_session_parameters WHERE parameter LIKE '%DATE%';

-- Test month recognition
SELECT TO_DATE('15-' || month_name || '-2024', 'DD-MON-YYYY') FROM month_list;

Prevention Guidelines

• Validate month ranges - Ensure 1-12 for numeric months
• Standardize formats - Use consistent date formats
• Handle languages - Set appropriate NLS_DATE_LANGUAGE
• Check spellings - Validate month name spelling
• Document formats - Clearly specify expected formats
• Test thoroughly - Include edge cases in testing