Invalid month index "${month}". Month index has to be betwee

Error message

Invalid month index "${month}". Month index has to be between 0 and 11.

What it means

DateFnsAdapter.createDate validates the month argument and throws when it falls outside 0–11 (date-fns/Material use zero-based months: 0=January, 11=December). The check runs in dev mode. This prevents silently rolling the date into the next year/month via Date overflow.

Source

Thrown at src/material-date-fns-adapter/adapter/date-fns-adapter.ts:136

  getFirstDayOfWeek(): number {
    return this.locale.options?.weekStartsOn ?? 0;
  }

  getNumDaysInMonth(date: Date): number {
    return getDaysInMonth(date);
  }

  clone(date: Date): Date {
    return new Date(date.getTime());
  }

  createDate(year: number, month: number, date: number): Date {
    if (typeof ngDevMode === 'undefined' || ngDevMode) {
      // Check for invalid month and date (except upper bound on date which we have to check after
      // creating the Date).
      if (month < 0 || month > 11) {
        throw Error(`Invalid month index "${month}". Month index has to be between 0 and 11.`);
      }

      if (date < 1) {
        throw Error(`Invalid date "${date}". Date has to be greater than 0.`);
      }
    }

    // Passing the year to the constructor causes year numbers <100 to be converted to 19xx.
    // To work around this we use `setFullYear` and `setHours` instead.
    const result = new Date();
    result.setFullYear(year, month, date);
    result.setHours(0, 0, 0, 0);

    // Check that the date wasn't above the upper bound for the month, causing the month to overflow
    if (result.getMonth() != month && (typeof ngDevMode === 'undefined' || ngDevMode)) {
      throw Error(`Invalid date "${date}" for month with index "${month}".`);
    }

View on GitHub (pinned to 0411926e7d)

Solutions

Subtract 1 from one-based month values before calling createDate.
Wrap month arithmetic with modulo: (month + 11) % 12 when decrementing.
Use date-fns helpers like setMonth/addMonths instead of raw indices.
Validate user-supplied month input against 1–12 before conversion.

Example fix

// before
this.adapter.createDate(2024, 12, 25); // intended December, throws
// after
this.adapter.createDate(2024, 11, 25); // zero-based: 11 = December

Defensive patterns

Strategy: validation

Validate before calling

if (!Number.isInteger(month) || month < 0 || month > 11) {
  throw new RangeError(`month must be 0-11, got ${month}`);
}
const d = adapter.createDate(year, month, date);

Type guard

function isValidMonthIndex(m: unknown): m is number {
  return typeof m === 'number' && Number.isInteger(m) && m >= 0 && m <= 11;
}

Try / catch

try {
  return this.adapter.createDate(year, month, day);
} catch (e) {
  if ((e as Error).message.startsWith('Invalid month index')) {
    return this.adapter.createDate(year, Math.min(Math.max(month, 0), 11), day);
  }
  throw e;
}

Prevention

Always convert one-based months (form/UI input) with month - 1
Use (m + 11) % 12 wrapping for previous-month arithmetic
Prefer adapter methods (addCalendarMonths, setMonth equivalents) over raw indices
Document zero-based convention wherever createDate is called

When it happens

Trigger: Calling adapter.createDate(year, month, date) with month < 0 or month > 11 — e.g. passing a human month number like 12 for December instead of 11, or -1 when computing previous month by naive subtraction.

Common situations: Converting from one-based user input (month pickers, forms) without subtracting 1; month arithmetic like currentMonth - 1 going below 0 without wrapping; mixing DateAdapter implementations with different conventions.

Related errors

AI-assisted analysis of angular/components@0411926e7d (2026-08-31). Data as JSON: /api/errors/bae2bdfb9c38685a. Report an issue: GitHub.