test: align day-boundary tests and docs with hijri-core's UTC-day contract

Convert all LOCAL-noon Date anchors (new Date(y, m, d, 12)) to UTC-explicit
anchors (new Date(Date.UTC(y, m-1, d))) in test.mjs and test-cjs.cjs.
Add UAQ default-engine round-trip regression suite (5 cases).
Extend FCNA round-trips; update vitest header comment.

README: add "Day boundaries and time zones" section explaining the UTC-day
contract, the correct pattern for zone-aware Luxon DateTimes, and ISO-string
parsing behaviour. Quick Start examples updated to use Date.UTC.

CHANGELOG: document inherited UTC-day fix under [Unreleased].

Lock-step dependency: requires hijri-core fix (commit 3419378,
branch fix/utc-day-boundary). Both packages release together per ADR-013.

Verified: TZ={UTC,America/New_York,Pacific/Auckland} × {test.mjs,
test-cjs.cjs, test-crossval.mjs, vitest} — all pass (88+26+120+15 tests).

CHANGELOG.md

@@ -7,6 +7,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Inherits hijri-core's UTC-day fix: `toHijri` with UTC-midnight Dates is now exact on all hosts
+  (previously, LOCAL date components were read, causing off-by-one errors west of UTC and on UTC+13+).
+- Round-trips (`toGregorian` then `toHijri`) are now exact for both the UAQ (default) and FCNA engines.
+- Tests updated to use `new Date(Date.UTC(...))` anchors throughout; UAQ engine round-trip regression
+  suite added. Lock-step release with hijri-core fix (commit 3419378).
+
 ## [3.0.1] - 2026-05-30
 
 ### Fixed

README.md

@@ -20,8 +20,8 @@ npm install luxon-hijri luxon hijri-core
 ```javascript
 import { toHijri, toGregorian, formatHijriDate } from 'luxon-hijri';
 
-// Gregorian to Hijri (Umm al-Qura, default)
-const h = toHijri(new Date(2023, 2, 23, 12));
+// Gregorian to Hijri (Umm al-Qura, default) — use Date.UTC for cross-host consistency
+const h = toHijri(new Date(Date.UTC(2023, 2, 23)));
 // { hy: 1444, hm: 9, hd: 1 }
 
 // Hijri to Gregorian
@@ -33,9 +33,33 @@ formatHijriDate({ hy: 1444, hm: 9, hd: 1 }, 'iEEEE, iD iMMMM iYYYY ioooo');
 // "Yawm al-Khamis, 1 Ramadan 1444 AH"
 
 // FCNA/ISNA calendar
-toHijri(new Date(2025, 2, 1, 12), { calendar: 'fcna' });
+toHijri(new Date(Date.UTC(2025, 2, 1)), { calendar: 'fcna' });
 ```
 
+## Day boundaries and time zones
+
+`toHijri(date)` reads the **UTC calendar day** of the Date you pass. `toGregorian(hy, hm, hd)` returns a Date at **UTC midnight** on the corresponding Gregorian day. Round-trips are therefore exact and produce identical results on any machine regardless of local timezone.
+
+**Converting a zone-aware Luxon DateTime.** Pass the DateTime's calendar fields, not `.toJSDate()`, unless the DateTime is already pinned to UTC:
+
+```javascript
+import { DateTime } from 'luxon';
+import { toHijri } from 'luxon-hijri';
+
+const dt = DateTime.now().setZone('America/New_York');
+
+// Correct — reads the calendar date in the DateTime's own zone
+const h = toHijri(new Date(Date.UTC(dt.year, dt.month - 1, dt.day)));
+
+// Wrong if dt is not UTC-anchored — toJSDate() produces local-zone midnight,
+// which may land on the previous UTC day for western timezones
+// const h = toHijri(dt.toJSDate());
+```
+
+**ISO string parsing.** `new Date("2025-03-01")` parses as UTC midnight — that is exactly the right input for a calendar-day conversion and will produce the correct Hijri date.
+
+Note: determining when the Hijri day begins at local sunset is out of scope for this library.
+
 ## TypeScript
 
 ```typescript

luxon-hijri.test.ts

@@ -0,0 +1,103 @@
+/**
+ * Purpose:    Vitest suite for luxon-hijri — conversion, formatting, and validation.
+ * Inputs:     Pure functions from src/index.ts. Requires luxon + hijri-core as peer deps.
+ * Outputs:    Vitest pass/fail assertions.
+ * Constraints: UAQ range 1318–1500 AH. toGregorian throws (not null) on invalid input.
+ *             toHijri reads the Date's UTC calendar day; pass UTC midnight or use
+ *             Date.UTC(year, month-1, day) for exact results on all hosts.
+ * Usage:      pnpm vitest run
+ * SOT:        packages.md — luxon-hijri row
+ */
+import { describe, it, expect } from "vitest";
+import {
+  toHijri,
+  toGregorian,
+  isValidHijriDate,
+  formatHijriDate,
+  hmLong,
+  hmMedium,
+  hmShort,
+} from "./src/index";
+
+// Anchor: toGregorian(1446, 9, 1) = 2025-03-01 midnight UTC
+// toHijri on noon 2025-03-01 reliably returns { hm: 9, hd: 1 }
+const RAMADAN_1446_NOON = new Date("2025-03-01T12:00:00Z");
+
+describe("toHijri", () => {
+  it("converts noon 2025-03-01 UTC to 1 Ramadan 1446", () => {
+    const result = toHijri(RAMADAN_1446_NOON);
+    expect(result).not.toBeNull();
+    expect(result!.hy).toBe(1446);
+    expect(result!.hm).toBe(9);
+    expect(result!.hd).toBe(1);
+  });
+
+  it("returns null for dates outside UAQ range", () => {
+    expect(toHijri(new Date("2100-01-01"))).toBeNull();
+  });
+
+  it("throws on an invalid Date", () => {
+    expect(() => toHijri(new Date("not-a-date"))).toThrow();
+  });
+});
+
+describe("toGregorian", () => {
+  it("converts 1 Ramadan 1446 to 2025-03-01 UTC midnight", () => {
+    const result = toGregorian(1446, 9, 1);
+    expect(result.toISOString()).toBe("2025-03-01T00:00:00.000Z");
+  });
+
+  it("throws on invalid Hijri date (out of range)", () => {
+    expect(() => toGregorian(1501, 1, 1)).toThrow("Invalid Hijri date");
+  });
+});
+
+describe("isValidHijriDate", () => {
+  it("returns true for 1 Ramadan 1446", () => {
+    expect(isValidHijriDate(1446, 9, 1)).toBe(true);
+  });
+
+  it("returns false for month 0", () => {
+    expect(isValidHijriDate(1446, 0, 1)).toBe(false);
+  });
+
+  it("returns false for day 31", () => {
+    expect(isValidHijriDate(1446, 1, 31)).toBe(false);
+  });
+});
+
+describe("formatHijriDate", () => {
+  const hijriDate = { hy: 1446, hm: 9, hd: 1 };
+
+  it("formats iYYYY-iMM-iDD correctly", () => {
+    expect(formatHijriDate(hijriDate, "iYYYY-iMM-iDD")).toBe("1446-09-01");
+  });
+
+  it("formats iMMMM as full month name Ramadan", () => {
+    expect(formatHijriDate(hijriDate, "iMMMM")).toBe("Ramadan");
+  });
+
+  it("formats iMMM as a non-empty medium month name", () => {
+    const result = formatHijriDate(hijriDate, "iMMM");
+    expect(typeof result).toBe("string");
+    expect(result.length).toBeGreaterThan(0);
+  });
+
+  it("throws RangeError on invalid month 0", () => {
+    expect(() => formatHijriDate({ hy: 1446, hm: 0, hd: 1 }, "iMMMM")).toThrow(RangeError);
+  });
+});
+
+describe("month name tables", () => {
+  it("hmLong index 8 is Ramadan", () => {
+    expect(hmLong[8]).toBe("Ramadan");
+  });
+
+  it("hmMedium has 12 entries", () => {
+    expect(hmMedium).toHaveLength(12);
+  });
+
+  it("hmShort has 12 entries", () => {
+    expect(hmShort).toHaveLength(12);
+  });
+});

package.json

@@ -40,7 +40,8 @@
     "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",
@@ -85,7 +86,8 @@
     "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",

test-cjs.cjs

@@ -43,11 +43,11 @@ describe('CJS core conversions', () => {
     assert.strictEqual(d.toISOString().slice(0, 10), '2023-03-23');
   });
   it('toHijri(2022-07-30) = 1 Muharram 1444', () => {
-    const h = toHijri(new Date(2022, 6, 30, 12));
+    const h = toHijri(new Date(Date.UTC(2022, 6, 30)));
     assert.deepEqual(h, { hy: 1444, hm: 1, hd: 1 });
   });
   it('toHijri(2023-03-23) = 1 Ramadan 1444', () => {
-    const h = toHijri(new Date(2023, 2, 23, 12));
+    const h = toHijri(new Date(Date.UTC(2023, 2, 23)));
     assert.deepEqual(h, { hy: 1444, hm: 9, hd: 1 });
   });
 });
@@ -106,7 +106,7 @@ describe('CJS FCNA calendar', () => {
     assert.strictEqual(d.toISOString().slice(0, 10), '2025-03-01');
   });
   it('2025-03-01 = 1 Ramadan 1446', () => {
-    const h = toHijri(new Date(2025, 2, 1, 12), FCNA);
+    const h = toHijri(new Date(Date.UTC(2025, 2, 1)), FCNA);
     assert.deepEqual(h, { hy: 1446, hm: 9, hd: 1 });
   });
   it('isValidHijriDate(1446, 9, 1) = true', () => {

test.mjs

@@ -95,23 +95,23 @@ describe('toGregorian - error cases', () => {
 
 describe('toHijri - known dates', () => {
   it('2022-07-30 = 1 Muharram 1444', () => {
-    const h = toHijri(new Date(2022, 6, 30, 12));
+    const h = toHijri(new Date(Date.UTC(2022, 6, 30)));
     assert.deepEqual(h, { hy: 1444, hm: 1, hd: 1 });
   });
   it('2023-03-23 = 1 Ramadan 1444', () => {
-    const h = toHijri(new Date(2023, 2, 23, 12));
+    const h = toHijri(new Date(Date.UTC(2023, 2, 23)));
     assert.deepEqual(h, { hy: 1444, hm: 9, hd: 1 });
   });
   it('2023-04-21 = 1 Shawwal 1444', () => {
-    const h = toHijri(new Date(2023, 3, 21, 12));
+    const h = toHijri(new Date(Date.UTC(2023, 3, 21)));
     assert.deepEqual(h, { hy: 1444, hm: 10, hd: 1 });
   });
   it('2024-07-07 = 1 Muharram 1446', () => {
-    const h = toHijri(new Date(2024, 6, 7, 12));
+    const h = toHijri(new Date(Date.UTC(2024, 6, 7)));
     assert.deepEqual(h, { hy: 1446, hm: 1, hd: 1 });
   });
   it('1900-04-30 = 1 Muharram 1318 (first table entry)', () => {
-    const h = toHijri(new Date(1900, 3, 30, 12));
+    const h = toHijri(new Date(Date.UTC(1900, 3, 30)));
     assert.deepEqual(h, { hy: 1318, hm: 1, hd: 1 });
   });
 });
@@ -274,15 +274,15 @@ describe('FCNA toGregorian', () => {
 
 describe('FCNA toHijri', () => {
   it('2025-03-01 = 1 Ramadan 1446', () => {
-    const h = toHijri(new Date(2025, 2, 1, 12), FCNA);
+    const h = toHijri(new Date(Date.UTC(2025, 2, 1)), FCNA);
     assert.deepEqual(h, { hy: 1446, hm: 9, hd: 1 });
   });
   it('2025-03-30 = 1 Shawwal 1446', () => {
-    const h = toHijri(new Date(2025, 2, 30, 12), FCNA);
+    const h = toHijri(new Date(Date.UTC(2025, 2, 30)), FCNA);
     assert.deepEqual(h, { hy: 1446, hm: 10, hd: 1 });
   });
   it('2024-03-11 = 1 Ramadan 1445', () => {
-    const h = toHijri(new Date(2024, 2, 11, 12), FCNA);
+    const h = toHijri(new Date(Date.UTC(2024, 2, 11)), FCNA);
     assert.deepEqual(h, { hy: 1445, hm: 9, hd: 1 });
   });
 });
@@ -312,6 +312,36 @@ describe('FCNA round-trips', () => {
   });
 });
 
+describe('UAQ round-trips (default engine)', () => {
+  it('1444/1/1 toGregorian then toHijri', () => {
+    const greg = toGregorian(1444, 1, 1);
+    const hijri = toHijri(greg);
+    assert.deepEqual(hijri, { hy: 1444, hm: 1, hd: 1 });
+  });
+  it('1444/9/1 toGregorian then toHijri', () => {
+    const greg = toGregorian(1444, 9, 1);
+    const hijri = toHijri(greg);
+    assert.deepEqual(hijri, { hy: 1444, hm: 9, hd: 1 });
+  });
+  it('1446/9/1 toGregorian then toHijri', () => {
+    const greg = toGregorian(1446, 9, 1);
+    const hijri = toHijri(greg);
+    assert.deepEqual(hijri, { hy: 1446, hm: 9, hd: 1 });
+  });
+  it('1318/1/1 toGregorian then toHijri (first table entry)', () => {
+    const greg = toGregorian(1318, 1, 1);
+    assert(greg instanceof Date);
+    const hijri = toHijri(greg);
+    assert.deepEqual(hijri, { hy: 1318, hm: 1, hd: 1 });
+  });
+  it('1500/12/29 toGregorian then toHijri (last table entry)', () => {
+    const greg = toGregorian(1500, 12, 29);
+    assert(greg instanceof Date);
+    const hijri = toHijri(greg);
+    assert.deepEqual(hijri, { hy: 1500, hm: 12, hd: 29 });
+  });
+});
+
 describe('FCNA isValidHijriDate', () => {
   it('1446/9/1 = true', () => assert.strictEqual(isValidHijriDate(1446, 9, 1, FCNA), true));
   it('month 0 = false', () => assert.strictEqual(isValidHijriDate(1446, 0, 1, FCNA), false));
@@ -329,7 +359,7 @@ describe('UAQ default regression', () => {
     assert.strictEqual(d.toISOString().slice(0, 10), '2025-03-01');
   });
   it('toHijri still works without options', () => {
-    const h = toHijri(new Date(2023, 2, 23, 12));
+    const h = toHijri(new Date(Date.UTC(2023, 2, 23)));
     assert.deepEqual(h, { hy: 1444, hm: 9, hd: 1 });
   });
   it('isValidHijriDate still works without options', () => {

vitest.config.ts

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