Recurrence Anchor Integrity

Checks a claimed series of recurring occurrences -- billing dates, reminders, recurring meetings -- against the anchor it was supposedly generated from, and reports MATCH, MISMATCH or NOT_DETERMINABLE for each one. No I/O, and no date generation of its own.

THE SILENT FAILURE, AND WHERE IT IS NOT. Every mainstream JS date library computes a single occurrence correctly when called directly from the anchor. That is not the finding -- it is the CONTROL this package's evidence starts from, measured in Node 26 against the real packages. The damage appears when a series is generated incrementally, each occurrence from the last rather than from the anchor: a DST transition or a month-end anchor shifts one step, and every occurrence after it inherits the shift. A monthly charge anchored on the 31st, a 09:00 meeting across a spring-forward boundary -- the dates stay perfectly valid and stop being the right dates.

IT VERIFIES SOMEONE ELSE'S ANSWER. This is deliberately not a generator. Your scheduler, your billing provider, or a model produced the series; this decides whether that series is consistent with its anchor, which is the check that is missing rather than another way to produce dates.

FAIL CLOSED. A series that cannot be checked -- an anchor with no zone, an interval the input never states -- returns NOT_DETERMINABLE naming the missing fact, never a confident MATCH.

WHAT VERIFICATION CHANGED. A rival implementation built from the README failed two tests, and both were the SUITE's fault, not the rival's: they pinned the exact WORDING of a rejection reason. A test that asserts a sentence breaks when you reword the sentence and proves nothing about behaviour. Those now assert the FACT -- that the reason names the offending field and its value -- so the suite still validates the code after you rewrite it.

VERIFIED: 134 tests, measured by running the suite, every mutation observed FAILING before restore.

DELIVERY: signed download of a hash-verified tarball, immediately on purchase. Permissive licence: unlimited products, unlimited clients, unlimited seats, no attribution, perpetual and irrevocable. One restriction, do not republish the source as source.

Interface

What you call, and what comes back. Types and signatures only — the implementation ships with the source.

  export function isGregorianLeapYear(year: number): boolean;
  export function daysInGregorianMonth(year: number, month1to12: number): number;
  export function addCalendarDays(date: CalendarDate, days: number): CalendarDate;
  export function addCalendarMonths( date: CalendarDate, months: number, convention: MonthEndConvention = "clamp-to-last-day" ): CalendarDate &;
  export function computeExpectedOccurrence( anchorDate: CalendarDate, anchorTime: WallClockTime, ianaZone: string, unit: IntervalUnit, count: number, cycle: number, monthEndConvention: MonthEndConvention ): ExpectedOccurrence;
  export function parseAnchor(anchor: unknown): ValidationResult<ParsedAnchor>;
  export function parseInterval(interval: unknown): ValidationResult<RecurrenceInterval>;
  export function parseMonthEndConvention(value: unknown): ValidationResult<MonthEndConvention>;
  export function parseOccurrencesArray(value: unknown): ValidationResult<unknown[]>;
  export function parseClaimedOccurrence(value: unknown, index: number): ValidationResult<ParsedClaim>;
  export function resolveAnchor( date: CalendarDate, time: WallClockTime, ianaZone: string ): ResolvedInstant;
  export function isRecognizedIanaZone(zone: string): boolean;

  export type MonthEndConvention = "clamp-to-last-day";
  export type ValidationResult<T> = { ok: true;
  export type IntervalUnit = "day" | "week" | "month" | "year";
  export type RecurrenceIntegrityStatus = "ALL_MATCH" | "MISMATCH_FOUND" | "NOT_DETERMINABLE";