TheAlgorithms/Java · error · IllegalArgumentException

Input date must be 10 characters long in the format MM-DD-YY

Error message

Input date must be 10 characters long in the format MM-DD-YYYY or MM/DD/YYYY.

What it means

Thrown by ZellersCongruence.calculateDay when the input is null or its length is not exactly 10. The parser uses fixed substring offsets (0-2, 3-5, 6-10) and fixed charAt positions (2, 5) for separators, so a non-10-character string would throw StringIndexOutOfBoundsException; this guard fails fast with a clear message instead.

Source

Thrown at src/main/java/com/thealgorithms/maths/ZellersCongruence.java:39

    // Private constructor to prevent instantiation
    private ZellersCongruence() {
    }

    /**
     * Calculates the day of the week for a given date using Zeller's Congruence.
     *
     * <p>The algorithm works for both Julian and Gregorian calendar dates. The input date must be
     * in the format "MM-DD-YYYY" or "MM/DD/YYYY".
     *
     * @param input the date in the format "MM-DD-YYYY" or "MM/DD/YYYY"
     * @return a string indicating the day of the week for the given date
     * @throws IllegalArgumentException if the input format is invalid, the date is invalid,
     *                                  or the year is out of range
     */
    public static String calculateDay(String input) {
        if (input == null || input.length() != 10) {
            throw new IllegalArgumentException("Input date must be 10 characters long in the format MM-DD-YYYY or MM/DD/YYYY.");
        }

        int month = parsePart(input.substring(0, 2), 1, 12, "Month must be between 1 and 12.");
        char sep1 = input.charAt(2);
        validateSeparator(sep1);

        int day = parsePart(input.substring(3, 5), 1, 31, "Day must be between 1 and 31.");
        char sep2 = input.charAt(5);
        validateSeparator(sep2);

        int year = parsePart(input.substring(6, 10), 46, 8499, "Year must be between 46 and 8499.");

        try {
            Objects.requireNonNull(LocalDate.of(year, month, day));
        } catch (DateTimeException e) {
            throw new IllegalArgumentException("Invalid date.", e);
        }
        if (month <= 2) {

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Normalize the input to exactly 10 characters: zero-pad month/day to 2 digits and year to 4 digits, e.g., via String.format("%02d-%02d-%04d", m, d, y).
2. Reject null/blank upstream with a dedicated validation error.
3. Parse with a LocalDate + DateTimeFormatter first, then reformat to the expected MM-DD-YYYY shape.

Example fix

// before
String day = ZellersCongruence.calculateDay(rawInput);

// after
String normalized = String.format("%02d-%02d-%04d", month, day, year);
String day = ZellersCongruence.calculateDay(normalized);

if (input == null || input.length() != 10) {
    throw new IllegalArgumentException("Date must be 10 chars: MM-DD-YYYY or MM/DD/YYYY");
}
String day = ZellersCongruence.calculateDay(input);

try {
    String d = ZellersCongruence.calculateDay(input);
} catch (IllegalArgumentException e) {
    errors.rejectValue("date", "format.invalid", e.getMessage());
}

When it happens

Trigger: Call calculateDay(null), calculateDay("1-1-2020"), calculateDay("01-01-20"), or any string whose length != 10 (including trailing whitespace or a missing leading zero).

Common situations: User-typed dates without zero-padding, locale-specific formats (D-M-YY), strings loaded from a CSV/DB column with inconsistent formatting, or a missing null check on optional input.

Related errors

• Invalid date.
• Month must be between 1 and 12.
• Day must be between 1 and 31.
• Year must be between 46 and 8499.
• Invalid numeric part: {part}

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/9a106cc3dace2e29. Report an issue: GitHub.