TheAlgorithms/Java · error · IllegalArgumentException

Invalid date.

Error message

Invalid date.

What it means

Thrown by ZellersCongruence.calculateDay when LocalDate.of(year, month, day) raises a DateTimeException. This is the second-stage check that catches semantically invalid dates that passed the coarse range checks: e.g., Feb 30, Apr 31, Feb 29 on a non-leap year. The original DateTimeException is chained as the cause.

Source

Thrown at src/main/java/com/thealgorithms/maths/ZellersCongruence.java:55

    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) {
            year -= 1;
            month += 12;
        }

        int century = year / 100;
        int yearOfCentury = year % 100;
        int t = (int) (2.6 * month - 5.39);
        int u = century / 4;
        int v = yearOfCentury / 4;
        int f = (int) Math.round((day + yearOfCentury + t + u + v - 2 * century) % 7.0);

        int correctedDay = (f + 7) % 7;

        return "The date " + input + " falls on a " + DAYS[correctedDay] + ".";
    }

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Validate the date with java.time.LocalDate.of(...) yourself before calling; if it constructs, the util will accept it.
2. Use a DatePicker/calendar control that disallows impossible days.
3. Catch IllegalArgumentException around the call and present a user-facing 'invalid calendar date' message.

Example fix

// before
String d = ZellersCongruence.calculateDay(input);

// after
// pre-validate so the error is yours, not the util's
LocalDate.of(year, month, day); // throws DateTimeException if invalid
String d = ZellersCongruence.calculateDay(input);

try {
    LocalDate.of(year, month, day); // throws DateTimeException if not a real calendar date
} catch (java.time.DateTimeException e) {
    throw new IllegalArgumentException("Not a real calendar date", e);
}
String d = ZellersCongruence.calculateDay(input);

try {
    String d = ZellersCongruence.calculateDay(input);
} catch (IllegalArgumentException e) {
    // could be format, range, OR invalid calendar date; chain has the cause
    return Result.invalid(e.getMessage());
}

When it happens

Trigger: Call calculateDay("02-30-2021"), calculateDay("04-31-2020"), calculateDay("02-29-2023") (2023 is not a leap year), or any date whose day exceeds the actual days-in-month.

Common situations: Accepting day values up to 31 unconditionally (the coarse check allows 1-31 for every month), user-entered dates without calendar awareness, or test fixtures that pick round numbers.

Related errors

• Input date must be 10 characters long in the format MM-DD-YY
• 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/c4648399cb753007. Report an issue: GitHub.