fix: convert the displayed calendar date in toHijri for hijri-core's UTC-day contract

toHijri() now passes Date.UTC(year, month, date) to hijri-core instead of the raw
instant from this.toDate(). Fixes wrong-Hijri-day results around UTC-midnight on
hosts east or west of UTC. Lock-step with hijri-core fix/utc-day-boundary.

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- `.toHijri()` now converts the calendar date the dayjs instance displays (via `Date.UTC(year, month, date)`) instead of passing the raw instant to hijri-core. Fixes wrong-Hijri-day results around UTC-midnight instants on hosts east or west of UTC. Lock-step with the unreleased hijri-core `fix/utc-day-boundary` fix.
+
 ## [1.0.2] - 2026-05-30
 
 ### Changed

README.md

@@ -36,6 +36,12 @@ dayjs.fromHijri(1444, 10, 1).format('YYYY-MM-DD'); // '2023-04-21'
 
 Full API reference, examples, and architecture notes are on the [GitHub Wiki](https://github.com/acamarata/dayjs-hijri-plus/wiki).
 
+## Day boundaries and time zones
+
+`.toHijri()` converts the calendar date the dayjs instance displays — the same date you would read off the screen — regardless of the host's system timezone or whether the dayjs `utc` plugin is active. A call like `dayjs('2025-03-01').toHijri()` always maps the 1st of March 2025, not whatever local instant that string resolves to in UTC.
+
+Religious start-of-day at sunset is out of scope. Sunset-aware day boundaries require external prayer-time data and are not handled here.
+
 ## Related
 
 - [hijri-core](https://github.com/acamarata/hijri-core): the zero-dependency Hijri calendar engine this plugin wraps

src/index.ts

@@ -167,7 +167,11 @@ const plugin: PluginFunc = (_option, dayjsClass, dayjsFactory) => {
   // ------------------------------------------------------------------ //
 
   dayjsClass.prototype.toHijri = function (opts?: ConversionOptions): HijriDate | null {
-    return toHijri(this.toDate(), opts);
+    // Build a UTC-noon Date from the calendar date this instance displays so
+    // that hijri-core's UTC-day contract reads the correct day regardless of
+    // the host timezone or whether the dayjs utc plugin is active.
+    // dayjs .month() is 0-based, matching Date.UTC's month parameter.
+    return toHijri(new Date(Date.UTC(this.year(), this.month(), this.date())), opts);
   };
 
   dayjsClass.prototype.isValidHijri = function (opts?: ConversionOptions): boolean {
@@ -262,9 +266,11 @@ const plugin: PluginFunc = (_option, dayjsClass, dayjsFactory) => {
     if (!greg) {
       throw new Error(`Invalid or out-of-range Hijri date: ${hy}/${hm}/${hd}`);
     }
-    // Construct from ISO date string to avoid timezone offset issues.
+    // Construct from an ISO date string (YYYY-MM-DD) so the result is the
-    // dayjsFactory(Date) interprets the Date in local time; a UTC-midnight Date
+    // Gregorian calendar day that corresponds to the Hijri date, at local
-    // in western timezones would resolve to the previous local day.
+    // midnight in whatever timezone the consumer uses. Passing a raw Date
+    // object to dayjsFactory() would interpret it as a UTC instant and could
+    // land on the previous local day for hosts west of UTC.
     const y = greg.getUTCFullYear();
     const mo = String(greg.getUTCMonth() + 1).padStart(2, "0");
     const dy = String(greg.getUTCDate()).padStart(2, "0");

test-cjs.cjs

@@ -59,3 +59,16 @@ describe('CJS: isValidHijri', () => {
     assert.equal(dayjs(D_RAMADAN_1444).isValidHijri(), true);
   });
 });
+
+describe('CJS: UTC-day boundary (regression)', () => {
+  it('dayjs("2025-03-01").toHijri() -> 1 Ramadan 1446', () => {
+    const h = dayjs('2025-03-01').toHijri();
+    assert.deepEqual(h, { hy: 1446, hm: 9, hd: 1 });
+  });
+
+  it('round-trip: fromHijri(1446,9,1) then toHijri() -> {1446,9,1}', () => {
+    const d = dayjs.fromHijri(1446, 9, 1);
+    const h = d.toHijri();
+    assert.deepEqual(h, { hy: 1446, hm: 9, hd: 1 });
+  });
+});

test.mjs

@@ -102,3 +102,18 @@ describe('isValidHijri', () => {
     assert.equal(valid, true);
   });
 });
+
+describe('UTC-day boundary (regression)', () => {
+  // dayjs("YYYY-MM-DD") parses as local midnight — timezone-invariant anchor
+  // for toHijri now that the adapter reads the displayed calendar date.
+  it('dayjs("2025-03-01").toHijri() -> 1 Ramadan 1446', () => {
+    const h = dayjs('2025-03-01').toHijri();
+    assert.deepEqual(h, { hy: 1446, hm: 9, hd: 1 });
+  });
+
+  it('round-trip: fromHijri(1446,9,1) then toHijri() -> {1446,9,1}', () => {
+    const d = dayjs.fromHijri(1446, 9, 1);
+    const h = d.toHijri();
+    assert.deepEqual(h, { hy: 1446, hm: 9, hd: 1 });
+  });
+});