Function assertValidDateFormat

packages/common/src/i18n/format_date.ts:152–168

* Asserts that the given date format is free from common mistakes. Throws an * error if one is found (except for the case of all "Y", in which case we just * log a warning). This should only be called in development mode.

(parts: string[])

 * log a warning).  This should only be called in development mode.
 */
function assertValidDateFormat(parts: string[]) {
  if (parts.some((part) => /^Y+$/.test(part)) && !parts.some((part) => /^w+$/.test(part))) {
    // "Y" indicates "week-based year", which differs from the actual calendar
    // year for a few days around Jan 1 most years.  Unless "w" is also
    // present (e.g. a date like "2024-W52") this is likely a mistake.  Users
    // probably meant "y" instead.
    const message = `Suspicious use of week-based year "Y" in date pattern "${parts.join(
      '',
    )}". Did you mean to use calendar year "y" instead?`;
    if (parts.length === 1) {
      // NOTE: allow "YYYY" with just a warning, since it's used in tests.
      console.error(formatRuntimeError(RuntimeErrorCode.SUSPICIOUS_DATE_FORMAT, message));
    } else {
      throw new RuntimeError(RuntimeErrorCode.SUSPICIOUS_DATE_FORMAT, message);
    }
  }
}

/**
 * Create a new Date object with the given date value, and the time set to midnight.