add opt-in anonymous telemetry (#1)

* add opt-in telemetry via @acamarata/telemetry (off by default)

* chore: update lockfile for @acamarata/telemetry devDep

* chore: fix prettier formatting on telemetry import

CHANGELOG.md

@@ -5,7 +5,15 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [Unreleased]
+## [1.0.4] - 2026-06-13
+
+### Fixed
+- Published package now includes dist/index.d.mts so ESM type resolution under node16/nodenext resolves the import condition.
+
+## [1.0.3] - 2026-06-10
+
+### Fixed
+- UAQ engine `toHijri` was reading local date components (`getFullYear/getMonth/getDate`) instead of UTC components, causing incorrect results on hosts west of UTC (e.g. `America/New_York`, `Pacific/Auckland`) when the input Date was a UTC-midnight value such as those returned by `toGregorian` or ISO date-only strings. `toHijri` now reads UTC calendar day components (`getUTCFullYear/getUTCMonth/getUTCDate`), matching the FCNA engine. **Behavior change:** on non-UTC hosts the converted Hijri day may shift to the UTC calendar day; round-trips via `toGregorian` are now exact on every machine.
 
 ## [1.0.2] - 2026-05-30
 

README.md

@@ -35,6 +35,27 @@ isValidHijriDate(1444, 9, 1); // true
 daysInHijriMonth(1444, 9);    // 29
 ```
 
+## Day boundaries and time zones
+
+hijri-core maps civil calendar days one-to-one (tabular UAQ, computed FCNA). The religious Hijri day beginning at sunset is intentionally out of scope.
+
+`toHijri` reads the input Date's UTC calendar day (`getUTCFullYear`, `getUTCMonth`, `getUTCDate`). `toGregorian` returns a UTC-midnight Date. This means round-trips are exact and results are identical on every machine regardless of local time zone:
+
+```typescript
+// Safe on any host — UTC-explicit construction
+const greg = toGregorian(1446, 9, 1);          // 2025-03-01T00:00:00.000Z
+const back = toHijri(greg!);                   // { hy: 1446, hm: 9, hd: 1 } — always exact
+
+// ISO date-only strings parse as UTC midnight — correct
+toHijri(new Date('2025-03-01'));               // { hy: 1446, hm: 9, hd: 1 }
+
+// For a local wall-clock date, construct explicitly in UTC
+toHijri(new Date(Date.UTC(2025, 2, 1)));       // { hy: 1446, hm: 9, hd: 1 }
+
+// Avoid local Date constructor for date-only conversions — breaks on UTC+13
+// toHijri(new Date(2025, 2, 1))  ← do NOT do this
+```
+
 ## Custom Calendars
 
 Implement `CalendarEngine` and call `registerCalendar('my-id', engine)`. Pass `{ calendar: 'my-id' }` to any conversion function.

TELEMETRY.md

@@ -0,0 +1,8 @@
+# Telemetry Disclosure
+
+This package supports opt-in anonymous usage telemetry via [`@acamarata/telemetry`](https://github.com/acamarata/telemetry).
+
+Telemetry is **off by default**. No data is sent unless you set `ACAMARATA_TELEMETRY=1`.
+
+Full disclosure (what is sent, where it goes, how to disable):
+[github.com/acamarata/telemetry/blob/main/TELEMETRY.md](https://github.com/acamarata/telemetry/blob/main/TELEMETRY.md)

eslint.config.mjs

@@ -5,12 +5,19 @@ import { typescript } from '@acamarata/eslint-config';
 
 export default [
   {
-    plugins: { '@typescript-eslint': tsPlugin },
-    languageOptions: { parser: tsParser },
+    ignores: ['dist/**', 'node_modules/**', 'coverage/**', 'docs/**'],
   },
-  ...typescript,
-  eslintConfigPrettier,
   {
-    ignores: ['dist/', 'node_modules/', '*.cjs', '*.mjs'],
+    files: ['src/**/*.ts'],
+    plugins: { '@typescript-eslint': tsPlugin },
+    languageOptions: {
+      parser: tsParser,
+      parserOptions: {
+        project: './tsconfig.json',
+        tsconfigRootDir: import.meta.dirname,
+      },
+    },
   },
+  ...typescript.map((cfg) => ({ ...cfg, files: ['src/**/*.ts'] })),
+  eslintConfigPrettier,
 ];

hijri-core.test.ts

@@ -0,0 +1,140 @@
+/**
+ * Purpose:    Vitest suite for hijri-core — conversion, validation, registry, and name exports.
+ * Inputs:     Pure functions from src/index.ts (no network; module-load registers uaq+fcna engines).
+ * Outputs:    Vitest pass/fail assertions.
+ * Constraints: UAQ calendar covers 1318–1500 AH (≈1900–2076 CE). toHijri interprets the input
+ *             Date by its UTC calendar day (getUTC* components); pass UTC-explicit Dates for
+ *             deterministic results. toGregorian returns UTC midnight, so round-trips are exact.
+ * Usage:      pnpm vitest run
+ * SOT:        packages.md — hijri-core row
+ */
+import { describe, it, expect } from "vitest";
+import {
+  toHijri,
+  toGregorian,
+  isValidHijriDate,
+  daysInHijriMonth,
+  listCalendars,
+  hmLong,
+  hwLong,
+} from "./src/index";
+
+describe("toGregorian (UAQ default)", () => {
+  it("converts 1 Ramadan 1446 to 2025-03-01 UTC midnight", () => {
+    const result = toGregorian(1446, 9, 1);
+    expect(result).not.toBeNull();
+    expect(result!.toISOString()).toBe("2025-03-01T00:00:00.000Z");
+  });
+
+  it("converts 1 Muharram 1446 to 2024-07-07 UTC midnight", () => {
+    const result = toGregorian(1446, 1, 1);
+    expect(result).not.toBeNull();
+    expect(result!.toISOString()).toBe("2024-07-07T00:00:00.000Z");
+  });
+
+  it("returns null for an out-of-range Hijri year (1501)", () => {
+    expect(toGregorian(1501, 1, 1)).toBeNull();
+  });
+});
+
+describe("toHijri (UAQ default)", () => {
+  it("converts 2025-03-01 noon UTC to 1 Ramadan 1446", () => {
+    // toGregorian(1446,9,1) = 2025-03-01 midnight; add 12h to stay on that Gregorian day
+    const noonOnRamadanStart = new Date("2025-03-01T12:00:00Z");
+    const result = toHijri(noonOnRamadanStart);
+    expect(result).not.toBeNull();
+    expect(result!.hy).toBe(1446);
+    expect(result!.hm).toBe(9);
+    expect(result!.hd).toBe(1);
+  });
+
+  it("returns null for a date outside UAQ range (year 2100)", () => {
+    expect(toHijri(new Date("2100-01-01T12:00:00Z"))).toBeNull();
+  });
+
+  it("throws on an invalid Date object", () => {
+    expect(() => toHijri(new Date("invalid"))).toThrow();
+  });
+});
+
+describe("isValidHijriDate", () => {
+  it("accepts a known valid date", () => {
+    expect(isValidHijriDate(1446, 9, 1)).toBe(true);
+  });
+
+  it("rejects month 13", () => {
+    expect(isValidHijriDate(1446, 13, 1)).toBe(false);
+  });
+
+  it("rejects day 0", () => {
+    expect(isValidHijriDate(1446, 1, 0)).toBe(false);
+  });
+});
+
+describe("daysInHijriMonth", () => {
+  it("returns 29 or 30 for a valid month", () => {
+    const days = daysInHijriMonth(1446, 9);
+    expect([29, 30]).toContain(days);
+  });
+});
+
+describe("registry", () => {
+  it("lists at least uaq and fcna after module load", () => {
+    const calendars = listCalendars();
+    expect(calendars).toContain("uaq");
+    expect(calendars).toContain("fcna");
+  });
+});
+
+describe("name tables", () => {
+  it("hmLong has 12 entries and index 8 is Ramadan", () => {
+    expect(hmLong).toHaveLength(12);
+    expect(hmLong[8]).toBe("Ramadan");
+  });
+
+  it("hwLong has 7 weekday entries", () => {
+    expect(hwLong).toHaveLength(7);
+  });
+});
+
+describe("day boundaries (UTC contract)", () => {
+  it("UAQ round-trip: toHijri(toGregorian(1446, 9, 1)) returns {hy:1446, hm:9, hd:1}", () => {
+    const greg = toGregorian(1446, 9, 1);
+    expect(greg).not.toBeNull();
+    const hijri = toHijri(greg!);
+    expect(hijri).not.toBeNull();
+    expect(hijri!.hy).toBe(1446);
+    expect(hijri!.hm).toBe(9);
+    expect(hijri!.hd).toBe(1);
+  });
+
+  it("toHijri(new Date('2025-03-01T00:00:00Z')) = 1 Ramadan 1446", () => {
+    const result = toHijri(new Date("2025-03-01T00:00:00Z"));
+    expect(result).not.toBeNull();
+    expect(result!.hy).toBe(1446);
+    expect(result!.hm).toBe(9);
+    expect(result!.hd).toBe(1);
+  });
+
+  it("last ms of 2025-02-28 UTC maps to the same Hijri day as noon on 2025-02-28 UTC", () => {
+    const lastMs = toHijri(new Date("2025-02-28T23:59:59.999Z"));
+    const noon = toHijri(new Date("2025-02-28T12:00:00Z"));
+    expect(lastMs).not.toBeNull();
+    expect(noon).not.toBeNull();
+    expect(lastMs!.hy).toBe(noon!.hy);
+    expect(lastMs!.hm).toBe(noon!.hm);
+    expect(lastMs!.hd).toBe(noon!.hd);
+    // and it is the day before 1 Ramadan 1446
+    expect(lastMs!.hm).not.toBe(9);
+  });
+
+  it("FCNA round-trip: toHijri(toGregorian(1446, 9, 1, {calendar:'fcna'}), {calendar:'fcna'}) is exact", () => {
+    const greg = toGregorian(1446, 9, 1, { calendar: "fcna" });
+    expect(greg).not.toBeNull();
+    const hijri = toHijri(greg!, { calendar: "fcna" });
+    expect(hijri).not.toBeNull();
+    expect(hijri!.hy).toBe(1446);
+    expect(hijri!.hm).toBe(9);
+    expect(hijri!.hd).toBe(1);
+  });
+});

package.json

@@ -1,6 +1,6 @@
 {
   "name": "hijri-core",
-  "version": "1.0.2",
+  "version": "1.0.4",
   "description": "Zero-dependency Hijri calendar engine with pluggable calendar support. Includes Umm al-Qura (UAQ) and FCNA/ISNA calendars. Extensible registry for custom calendars.",
   "author": "Aric Camarata",
   "license": "MIT",
@@ -34,13 +34,14 @@
     "typecheck": "tsc --noEmit",
     "pretest": "tsup",
     "test": "node --test test.mjs && node --test test-cjs.cjs",
-    "prepublishOnly": "tsup",
+    "prepack": "pnpm run build",
     "lint": "eslint .",
     "format": "prettier --write src/",
     "format:check": "prettier --check src/",
     "coverage": "c8 --reporter=lcov --reporter=text node test.mjs",
     "docs": "typedoc --out .github/wiki/api src/index.ts",
-    "postbuild": "cp dist/index.d.ts dist/index.d.mts"
+    "postbuild": "cp dist/index.d.ts dist/index.d.mts",
+    "test:vitest": "vitest run"
   },
   "keywords": [
     "hijri",
@@ -57,9 +58,12 @@
   "devDependencies": {
     "@acamarata/eslint-config": "^0.1.0",
     "@acamarata/prettier-config": "^0.1.0",
+    "@acamarata/telemetry": "^0.1.0",
     "@acamarata/tsconfig": "^0.1.0",
     "@eslint/js": "^10.0.1",
     "@types/node": "^22.15.0",
+    "@typescript-eslint/eslint-plugin": "^8.56.1",
+    "@typescript-eslint/parser": "^8.56.1",
     "c8": "^10.1.0",
     "eslint": "^10.0.3",
     "eslint-config-prettier": "^10.1.8",
@@ -68,7 +72,8 @@
     "typedoc": "^0.28.19",
     "typedoc-plugin-markdown": "^4.11.0",
     "typescript": "^5.5.0",
-    "typescript-eslint": "^8.56.1"
+    "typescript-eslint": "^8.56.1",
+    "vitest": "^2.1.9"
   },
   "publishConfig": {
     "access": "public",

src/data/hDates.ts

@@ -1,4 +1,4 @@
-import type { HijriYearRecord } from '../types';
+import type { HijriYearRecord } from "../types";
 
 // Umm al-Qura reference table: Hijri years 1318-1501.
 // Each entry records the 1 Muharram Gregorian date and a 12-bit days-per-month

src/engines/fcna.ts

@@ -7,9 +7,9 @@
 // New moon times come from Jean Meeus, Astronomical Algorithms (2nd ed.),
 // Chapter 49, accurate to within a few minutes for 1000-3000 CE.
 
-import { hDatesTable } from '../data/hDates';
-import { MS_PER_DAY, MONTHS_PER_YEAR } from '../constants';
-import type { CalendarEngine, HijriDate } from '../types';
+import { hDatesTable } from "../data/hDates";
+import { MS_PER_DAY, MONTHS_PER_YEAR } from "../constants";
+import type { CalendarEngine, HijriDate } from "../types";
 
 // ─── Constants ───────────────────────────────────────────────────────────────
 
@@ -211,7 +211,7 @@ function fcnaDaysInMonth(hy: number, hm: number): number {
 
 function fcnaToHijri(gregorianDate: Date): HijriDate | null {
   if (!(gregorianDate instanceof Date) || isNaN(gregorianDate.getTime())) {
-    throw new Error('Invalid Gregorian date');
+    throw new Error("Invalid Gregorian date");
   }
 
   // FCNA criterion is UTC-based, so UTC date components ensure correct round-trips.
@@ -271,7 +271,7 @@ function fcnaIsValid(hy: number, hm: number, hd: number): boolean {
 // ─── Engine export ────────────────────────────────────────────────────────────
 
 export const fcnaEngine: CalendarEngine = {
-  id: 'fcna',
+  id: "fcna",
   toHijri: fcnaToHijri,
   toGregorian: fcnaToGregorian,
   isValid: fcnaIsValid,

src/engines/uaq.ts

@@ -4,9 +4,9 @@
 // (Gregorian 1900-2076). Each entry records the Gregorian date of 1 Muharram and
 // a 12-bit days-per-month bitmask. Dates outside that window return null.
 
-import { hDatesTable } from '../data/hDates';
-import { MS_PER_DAY, MONTHS_PER_YEAR } from '../constants';
-import type { CalendarEngine, HijriDate, HijriYearRecord } from '../types';
+import { hDatesTable } from "../data/hDates";
+import { MS_PER_DAY, MONTHS_PER_YEAR } from "../constants";
+import type { CalendarEngine, HijriDate, HijriYearRecord } from "../types";
 
 /**
  * Binary search for a Hijri year entry in the UAQ table.
@@ -34,14 +34,17 @@ function findYearEntry(hy: number): HijriYearRecord | null {
   return null;
 }
 
-// toHijri uses local date components (getFullYear, getMonth, getDate) so that
-// the calendar-date lookup is timezone-safe regardless of the host environment.
+// toHijri interprets the input Date by its UTC calendar day (getUTC* components),
+// matching the FCNA engine and the UTC-midnight Dates returned by toGregorian.
+// This makes conversions host-timezone-independent and round-trips exact:
+//   toHijri(toGregorian(hy, hm, hd)) === { hy, hm, hd }  on any machine.
+// To convert a local wall-clock date, pass new Date(Date.UTC(y, m - 1, d)).
 function uaqToHijri(date: Date): HijriDate | null {
   if (!(date instanceof Date) || isNaN(date.getTime())) {
-    throw new Error('Invalid Gregorian date');
+    throw new Error("Invalid Gregorian date");
   }
 
-  const inputUtc = Date.UTC(date.getFullYear(), date.getMonth(), date.getDate());
+  const inputUtc = Date.UTC(date.getUTCFullYear(), date.getUTCMonth(), date.getUTCDate());
 
   // Binary search: find the last table entry whose Gregorian start date <= input.
   let lo = 0;
@@ -126,7 +129,7 @@ function uaqDaysInMonth(hy: number, hm: number): number {
 }
 
 export const uaqEngine: CalendarEngine = {
-  id: 'uaq',
+  id: "uaq",
   toHijri: uaqToHijri,
   toGregorian: uaqToGregorian,
   isValid: uaqIsValid,

src/index.ts

@@ -1,32 +1,32 @@
 // Built-in engines are registered at module load so that 'uaq' and 'fcna' are
 // available immediately on import. This module-level side effect is intentional
 // and documented in the sideEffects field of package.json.
-import { uaqEngine } from './engines/uaq';
-import { fcnaEngine } from './engines/fcna';
-import { registerCalendar } from './registry';
+import { uaqEngine } from "./engines/uaq";
+import { fcnaEngine } from "./engines/fcna";
+import { registerCalendar } from "./registry";
 
-registerCalendar('uaq', uaqEngine);
-registerCalendar('fcna', fcnaEngine);
+registerCalendar("uaq", uaqEngine);
+registerCalendar("fcna", fcnaEngine);
 
 // Registry
-export { registerCalendar, getCalendar, listCalendars } from './registry';
+export { registerCalendar, getCalendar, listCalendars } from "./registry";
 
 // Constants
-export { MS_PER_DAY, MONTHS_PER_YEAR } from './constants';
+export { MS_PER_DAY, MONTHS_PER_YEAR } from "./constants";
 
 // Types
-export type { HijriDate, HijriYearRecord, CalendarEngine, ConversionOptions } from './types';
+export type { HijriDate, HijriYearRecord, CalendarEngine, ConversionOptions } from "./types";
 
 // Data
-export { hDatesTable } from './data/hDates';
+export { hDatesTable } from "./data/hDates";
 
 // Names
-export { hmLong, hmMedium, hmShort } from './names/months';
-export { hwLong, hwShort, hwNumeric } from './names/weekdays';
+export { hmLong, hmMedium, hmShort } from "./names/months";
+export { hwLong, hwShort, hwNumeric } from "./names/weekdays";
 
 // Convenience wrappers
-import { getCalendar } from './registry';
-import type { HijriDate, ConversionOptions } from './types';
+import { getCalendar } from "./registry";
+import type { HijriDate, ConversionOptions } from "./types";
 
 /**
  * Convert a Gregorian date to a Hijri date.
@@ -34,6 +34,14 @@ import type { HijriDate, ConversionOptions } from './types';
  * Uses the UAQ (Umm al-Qura) calendar by default. Pass `{ calendar: 'fcna' }`
  * or any registered calendar name via options to use a different engine.
  *
+ * **Time-zone contract:** the Date is interpreted by its UTC calendar day
+ * (`getUTCFullYear`, `getUTCMonth`, `getUTCDate`). `toGregorian` returns a
+ * UTC-midnight Date, so round-trips are exact and results are identical on
+ * every host regardless of its local time zone.
+ *
+ * To convert a local wall-clock date, pass `new Date(Date.UTC(y, m - 1, d))`.
+ * Note that `new Date("2025-03-01")` parses as UTC midnight, which is correct.
+ *
  * @param date - a valid JavaScript Date object
  * @param options - conversion options (calendar engine selection)
  * @returns the corresponding Hijri date, or null if the date is out of range
@@ -41,9 +49,9 @@ import type { HijriDate, ConversionOptions } from './types';
  */
 export function toHijri(date: Date, options?: ConversionOptions): HijriDate | null {
   if (!(date instanceof Date) || isNaN(date.getTime())) {
-    throw new Error('Invalid Gregorian date');
+    throw new Error("Invalid Gregorian date");
   }
-  return getCalendar(options?.calendar ?? 'uaq').toHijri(date);
+  return getCalendar(options?.calendar ?? "uaq").toHijri(date);
 }
 
 /**
@@ -63,7 +71,7 @@ export function toGregorian(
   hd: number,
   options?: ConversionOptions,
 ): Date | null {
-  return getCalendar(options?.calendar ?? 'uaq').toGregorian(hy, hm, hd);
+  return getCalendar(options?.calendar ?? "uaq").toGregorian(hy, hm, hd);
 }
 
 /**
@@ -81,7 +89,7 @@ export function isValidHijriDate(
   hd: number,
   options?: ConversionOptions,
 ): boolean {
-  return getCalendar(options?.calendar ?? 'uaq').isValid(hy, hm, hd);
+  return getCalendar(options?.calendar ?? "uaq").isValid(hy, hm, hd);
 }
 
 /**
@@ -94,5 +102,14 @@ export function isValidHijriDate(
  * @throws {RangeError} if the month or year is out of range
  */
 export function daysInHijriMonth(hy: number, hm: number, options?: ConversionOptions): number {
-  return getCalendar(options?.calendar ?? 'uaq').daysInMonth(hy, hm);
+  return getCalendar(options?.calendar ?? "uaq").daysInMonth(hy, hm);
 }
+
+// ── Opt-in anonymous telemetry ────────────────────────────────────────────────
+// Off by default. Enable: ACAMARATA_TELEMETRY=1
+// What is sent + how to disable: https://github.com/acamarata/telemetry/blob/main/TELEMETRY.md
+import("@acamarata/telemetry")
+  .then(({ track }) => track("load", { package: "hijri-core", version: "1.0.4" }))
+  .catch(() => {
+    // telemetry not installed or disabled — that's fine
+  });

src/names/months.ts

@@ -11,18 +11,18 @@
  * const month = hmLong[hijriDate.hm - 1]; // "Ramadan"
  */
 export const hmLong = [
-  'Muharram', // 1
-  'Safar', // 2
+  "Muharram", // 1
+  "Safar", // 2
   "Rabi'l Awwal", // 3
   "Rabi'l Thani", // 4
-  'Jumadal Awwal', // 5
-  'Jumadal Thani', // 6
-  'Rajab', // 7
+  "Jumadal Awwal", // 5
+  "Jumadal Thani", // 6
+  "Rajab", // 7
   "Sha'ban", // 8
-  'Ramadan', // 9
-  'Shawwal', // 10
+  "Ramadan", // 9
+  "Shawwal", // 10
   "Dhul Qi'dah", // 11
-  'Dhul Hijjah', // 12
+  "Dhul Hijjah", // 12
 ];
 
 /**
@@ -35,18 +35,18 @@ export const hmLong = [
  * const label = hmMedium[hijriDate.hm - 1]; // "Ramadan"
  */
 export const hmMedium = [
-  'Muharram',
-  'Safar',
-  'Rabi1',
-  'Rabi2',
-  'Jumada1',
-  'Jumada2',
-  'Rajab',
-  'Shaban',
-  'Ramadan',
-  'Shawwal',
-  'Dhul-Qidah',
-  'Dhul-Hijjah',
+  "Muharram",
+  "Safar",
+  "Rabi1",
+  "Rabi2",
+  "Jumada1",
+  "Jumada2",
+  "Rajab",
+  "Shaban",
+  "Ramadan",
+  "Shawwal",
+  "Dhul-Qidah",
+  "Dhul-Hijjah",
 ];
 
 /**
@@ -59,16 +59,16 @@ export const hmMedium = [
  * const abbr = hmShort[hijriDate.hm - 1]; // "Ram"
  */
 export const hmShort = [
-  'Muh',
-  'Saf',
-  'Ra1',
-  'Ra2',
-  'Ju1',
-  'Ju2',
-  'Raj',
-  'Shb',
-  'Ram',
-  'Shw',
-  'DhQ',
-  'DhH',
+  "Muh",
+  "Saf",
+  "Ra1",
+  "Ra2",
+  "Ju1",
+  "Ju2",
+  "Raj",
+  "Shb",
+  "Ram",
+  "Shw",
+  "DhQ",
+  "DhH",
 ];

src/names/weekdays.ts

@@ -11,13 +11,13 @@
  * const dayName = hwLong[gregorianDate.getDay()]; // "Yawm al-Jum`a"
  */
 export const hwLong = [
-  'Yawm al-Ahad', // Sunday
-  'Yawm al-Ithnayn', // Monday
+  "Yawm al-Ahad", // Sunday
+  "Yawm al-Ithnayn", // Monday
   "Yawm ath-Thulatha'", // Tuesday
   "Yawm al-Arba`a'", // Wednesday
-  'Yawm al-Khamis', // Thursday
-  'Yawm al-Jum`a', // Friday
-  'Yawm as-Sabt', // Saturday
+  "Yawm al-Khamis", // Thursday
+  "Yawm al-Jum`a", // Friday
+  "Yawm as-Sabt", // Saturday
 ];
 
 /**
@@ -30,13 +30,13 @@ export const hwLong = [
  * const abbr = hwShort[gregorianDate.getDay()]; // "Jum`a"
  */
 export const hwShort = [
-  'Ahad', // Sunday
-  'Ithn', // Monday
-  'Thul', // Tuesday
-  'Arba', // Wednesday
-  'Kham', // Thursday
-  'Jum`a', // Friday
-  'Sabt', // Saturday
+  "Ahad", // Sunday
+  "Ithn", // Monday
+  "Thul", // Tuesday
+  "Arba", // Wednesday
+  "Kham", // Thursday
+  "Jum`a", // Friday
+  "Sabt", // Saturday
 ];
 
 /**

src/registry.ts

@@ -1,4 +1,4 @@
-import type { CalendarEngine } from './types';
+import type { CalendarEngine } from "./types";
 
 const _engines = new Map<string, CalendarEngine>();
 
@@ -25,7 +25,7 @@ export function registerCalendar(name: string, engine: CalendarEngine): void {
 export function getCalendar(name: string): CalendarEngine {
   const engine = _engines.get(name);
   if (!engine) {
-    const available = listCalendars().join(', ');
+    const available = listCalendars().join(", ");
     throw new Error(
       `Unknown Hijri calendar: "${name}". Available: ${available}. Register custom calendars with registerCalendar().`,
     );

test-cjs.cjs

@@ -61,7 +61,7 @@ describe('CJS UAQ conversions', () => {
     assert.equal(d.toISOString().slice(0, 10), '2025-03-01');
   });
   it('toHijri: 2023-03-23 = 1444/9/1', () => {
-    const h = toHijri(new Date(2023, 2, 23, 12));
+    const h = toHijri(new Date(Date.UTC(2023, 2, 23, 12)));
     assert.ok(h !== null);
     assert.equal(h.hy, 1444);
     assert.equal(h.hm, 9);

test.mjs

@@ -97,19 +97,28 @@ describe('UAQ toGregorian', () => {
 
 describe('UAQ toHijri', () => {
   it('2023-03-23 = 1444/9/1', () => {
-    const h = toHijri(new Date(2023, 2, 23, 12));
+    const h = toHijri(new Date(Date.UTC(2023, 2, 23, 12)));
     assert.ok(h !== null);
     assert.equal(h.hy, 1444);
     assert.equal(h.hm, 9);
     assert.equal(h.hd, 1);
   });
   it('2025-03-01 = 1446/9/1', () => {
-    const h = toHijri(new Date(2025, 2, 1, 12));
+    const h = toHijri(new Date(Date.UTC(2025, 2, 1, 12)));
     assert.ok(h !== null);
     assert.equal(h.hy, 1446);
     assert.equal(h.hm, 9);
     assert.equal(h.hd, 1);
   });
+  it('UAQ round-trip: toHijri(toGregorian(1446, 9, 1)) = 1446/9/1', () => {
+    const greg = toGregorian(1446, 9, 1);
+    assert.ok(greg instanceof Date);
+    const hijri = toHijri(greg);
+    assert.ok(hijri !== null);
+    assert.equal(hijri.hy, 1446);
+    assert.equal(hijri.hm, 9);
+    assert.equal(hijri.hd, 1);
+  });
 });
 
 // ─── UAQ isValidHijriDate ───────────────────────────────────────────────────

vitest.config.ts

@@ -0,0 +1,9 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: "node",
+    include: ["hijri-core.test.ts"],
+  },
+});