fix: interpret DateTime by UTC calendar day for exact round-trips

toHijri in both UAQ and FCNA engines read .year/.month/.day from the
input DateTime — local calendar components — then passed them to
DateTime.utc(). On hosts west of UTC a UTC-midnight local DateTime
resolves to the previous local day, causing toHijri(toGregorian(y,m,d))
to return the wrong Hijri day.

Fix: call date.toUtc() first in both engines before extracting calendar
components. This is symmetric with toGregorian which always returns
DateTime.utc(). Adds 3 regression tests covering the round-trip and the
local-DateTime-on-UTC-west-host case.

CHANGELOG.md

@@ -5,6 +5,16 @@ All notable changes to this project will be documented in this file.
 The format follows [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 This project adheres to [Semantic Versioning](https://semver.org/).
 
+## [Unreleased]
+
+### Fixed
+
+- `toHijri` now normalizes the input `DateTime` to its UTC calendar day before
+  lookup (via `date.toUtc()`), matching the UTC-midnight contract of `toGregorian`.
+  Previously, passing a local `DateTime` on a host west of UTC could return the
+  previous Hijri day, breaking `toHijri(toGregorian(y, m, d))` round-trips.
+  Applies to both the UAQ and FCNA engines.
+
 ## [1.0.0] - 2026-05-25
 
 ### Added

lib/src/engines/fcna.dart

@@ -234,12 +234,15 @@ int _fcnaDaysInMonth(int hy, int hm) {
 // ---- FCNA Gregorian -> Hijri ----
 
 HijriDate? _fcnaToHijri(DateTime date) {
-  // FCNA criterion is UTC-based, so UTC date components ensure correct round-trips.
+  // Normalize to UTC calendar day so the result is deterministic regardless of
+  // the host timezone and whether the caller passes a UTC or local DateTime.
+  // This is symmetric with toGregorian, which always returns DateTime.utc().
+  final d = date.toUtc();
   final inputMs =
       DateTime.utc(
-        date.year,
-        date.month,
-        date.day,
+        d.year,
+        d.month,
+        d.day,
       ).millisecondsSinceEpoch.toDouble();
 
   final kApprox = _utcMsToKApprox(inputMs - 15 * msPerDay);

lib/src/engines/uaq.dart

@@ -36,7 +36,11 @@ int _dateUtcMs(int year, int month, int day) {
 }
 
 HijriDate? _uaqToHijri(DateTime date) {
-  final inputUtc = _dateUtcMs(date.year, date.month, date.day);
+  // Normalize to UTC calendar day so the result is deterministic regardless of
+  // the host timezone and whether the caller passes a UTC or local DateTime.
+  // This is symmetric with toGregorian, which always returns DateTime.utc().
+  final d = date.toUtc();
+  final inputUtc = _dateUtcMs(d.year, d.month, d.day);
 
   // Binary search: find the last table entry whose Gregorian start date <= input.
   int lo = 0;

test/hijri_core_test.dart

@@ -93,6 +93,41 @@ void main() {
     });
   });
 
+  // ---- UAQ round-trips (UTC day boundary) ----
+
+  group('UAQ round-trips', () {
+    test('toHijri(toGregorian(1446,9,1)) == 1446/9/1', () {
+      final greg = toGregorian(1446, 9, 1);
+      expect(greg, isNotNull);
+      final h = toHijri(greg!);
+      expect(h, isNotNull);
+      expect(h!.hy, equals(1446));
+      expect(h.hm, equals(9));
+      expect(h.hd, equals(1));
+    });
+    test('local DateTime with same UTC day round-trips correctly', () {
+      // toGregorian returns DateTime.utc(); converting it to local must still
+      // produce the correct Hijri day (regression for UTC-west hosts).
+      final greg = toGregorian(1446, 9, 1);
+      expect(greg, isNotNull);
+      final localGreg = greg!.toLocal();
+      final h = toHijri(localGreg);
+      expect(h, isNotNull);
+      expect(h!.hy, equals(1446));
+      expect(h.hm, equals(9));
+      expect(h.hd, equals(1));
+    });
+    test('toHijri(toGregorian(1318,1,1)) == 1318/1/1 (table lower bound)', () {
+      final greg = toGregorian(1318, 1, 1);
+      expect(greg, isNotNull);
+      final h = toHijri(greg!);
+      expect(h, isNotNull);
+      expect(h!.hy, equals(1318));
+      expect(h.hm, equals(1));
+      expect(h.hd, equals(1));
+    });
+  });
+
   // ---- UAQ isValid ----
 
   group('UAQ isValid', () {