fix: normalize date to UTC calendar day so prayer times are host-timezone-independent

getTimes now constructs civDate = DateTime.utc(date.year, date.month, date.day, 12, 0, 0)
at entry and passes it to both getSpa and getAngles. Previously a local DateTime in a
positive-UTC-offset zone (e.g. UTC+12) would reach getSpa.toUtc() as the previous UTC
calendar day, shifting all prayer times by ~24 h. Regression tests added for UTC/local
DateTime consistency across UTC, EDT, and Auckland host timezones (27 tests, all pass).

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
+
+- Prayer times are now host-timezone-independent. `getTimes` normalizes the
+  caller's `date` to a stable UTC-noon reference (`DateTime.utc(y, m, d, 12)`)
+  before passing it to `getSpa` and all astronomical calculations. Previously,
+  a local `DateTime(2024, 3, 15)` in a UTC+12 zone would reach `getSpa` as
+  UTC March 14, shifting all times by one civil day.
+
 ## [1.0.0] - 2026-05-25
 
 ### Added

lib/src/angles.dart

@@ -68,6 +68,8 @@ TwilightAngles getAngles(
   double pressure = 1013.25,
 }) {
   // 1. Solar ephemeris at UTC noon of the given date.
+  //    date.year/month/day are read without TZ conversion so they reflect the
+  //    caller's expressed civil date for both local and UTC DateTime inputs.
   final noonDate = DateTime.utc(date.year, date.month, date.day, 12, 0, 0);
   final jd = toJulianDate(noonDate);
   final eph = solarEphemeris(jd);

lib/src/get_times.dart

@@ -33,9 +33,17 @@ PrayerTimes getTimes(
   double pressure = 1013.25,
   bool hanafi = false,
 }) {
+  // Normalize to a stable UTC-noon DateTime for this civil calendar date.
+  // Reading date.year/month/day directly (without TZ conversion) preserves
+  // the caller's expressed date regardless of whether they passed a local or
+  // UTC DateTime. Constructing UTC noon removes host-timezone influence from
+  // every downstream Julian-Day computation and aligns getSpa with the
+  // Meeus/MSC calculations (which all need the same civil date).
+  final civDate = DateTime.utc(date.year, date.month, date.day, 12, 0, 0);
+
   // 1. Compute dynamic twilight angles.
   final tw = getAngles(
-    date,
+    civDate,
     lat,
     lng,
     elevation: elevation,
@@ -49,8 +57,11 @@ PrayerTimes getTimes(
   final ishaZenith = 90 + tw.ishaAngle;
 
   // 3. Run SPA for solar position + custom twilight times.
+  //    Pass civDate (UTC noon) so SPA receives a deterministic UTC instant
+  //    and the date component it extracts (via date.toUtc()) is always the
+  //    intended civil day, independent of host timezone.
   final spaData = getSpa(
-    date,
+    civDate,
     lat,
     lng,
     tz,
@@ -70,9 +81,8 @@ PrayerTimes getTimes(
   final dhuhrTime = noonTime + 2.5 / 60;
 
   // 4. Solar declination for Asr (Meeus formula, accurate to ~0.01°).
-  final jd = toJulianDate(
+  //    civDate already is UTC noon of the civil date.
-    DateTime.utc(date.year, date.month, date.day, 12, 0, 0),
+  final jd = toJulianDate(civDate);
-  );
   final eph = solarEphemeris(jd);
 
   // 5. Asr time.

test/pray_calc_dart_test.dart

@@ -190,4 +190,57 @@ void main() {
       expect(result.angles[0].sunset, lessThan(result.angles[1].sunset));
     });
   });
+
+  group('UTC day-boundary regression — civil-date consistency', () {
+    // getTimes normalizes the input date to UTC noon of the expressed civil
+    // date before passing it to getSpa and all astronomical calculations.
+    // This ensures that a local DateTime and a UTC DateTime that both express
+    // "March 15" (via their .year/.month/.day fields) produce identical prayer
+    // times, regardless of what the host machine timezone is.
+
+    test('local DateTime and UTC noon for same civil date produce identical times', () {
+      // Both express 2024-03-15: local midnight and UTC noon share year/month/day = 2024/3/15
+      final local = DateTime(2024, 3, 15);
+      final utcNoon = DateTime.utc(2024, 3, 15, 12, 0, 0);
+
+      final timesLocal = getTimes(local, 40.7128, -74.0060, -5.0);
+      final timesNoon = getTimes(utcNoon, 40.7128, -74.0060, -5.0);
+
+      // After normalization both resolve to the same UTC noon reference point
+      expect(timesNoon.fajr, closeTo(timesLocal.fajr, 0.0001));
+      expect(timesNoon.sunrise, closeTo(timesLocal.sunrise, 0.0001));
+      expect(timesNoon.dhuhr, closeTo(timesLocal.dhuhr, 0.0001));
+      expect(timesNoon.asr, closeTo(timesLocal.asr, 0.0001));
+      expect(timesNoon.maghrib, closeTo(timesLocal.maghrib, 0.0001));
+      expect(timesNoon.isha, closeTo(timesLocal.isha, 0.0001));
+    });
+
+    test('UTC midnight and UTC noon for same civil date produce identical times', () {
+      final utcMidnight = DateTime.utc(2024, 3, 15, 0, 0, 0);
+      final utcNoon = DateTime.utc(2024, 3, 15, 12, 0, 0);
+
+      final timesMid = getTimes(utcMidnight, 40.7128, -74.0060, -5.0);
+      final timesNoon = getTimes(utcNoon, 40.7128, -74.0060, -5.0);
+
+      expect(timesNoon.fajr, closeTo(timesMid.fajr, 0.0001));
+      expect(timesNoon.sunrise, closeTo(timesMid.sunrise, 0.0001));
+      expect(timesNoon.dhuhr, closeTo(timesMid.dhuhr, 0.0001));
+      expect(timesNoon.asr, closeTo(timesMid.asr, 0.0001));
+      expect(timesNoon.maghrib, closeTo(timesMid.maghrib, 0.0001));
+      expect(timesNoon.isha, closeTo(timesMid.isha, 0.0001));
+    });
+
+    test('angles are identical for local vs UTC noon for the same civil date', () {
+      final local = DateTime(2024, 6, 15);
+      final utcNoon = DateTime.utc(2024, 6, 15, 12, 0, 0);
+
+      // getAngles receives civDate (already UTC noon from getTimes) but can
+      // also be called directly — test that same civil date gives same angles.
+      final anglesLocal = getAngles(local, 40.7128, -74.0060);
+      final anglesUtcNoon = getAngles(utcNoon, 40.7128, -74.0060);
+
+      expect(anglesUtcNoon.fajrAngle, closeTo(anglesLocal.fajrAngle, 0.001));
+      expect(anglesUtcNoon.ishaAngle, closeTo(anglesLocal.ishaAngle, 0.001));
+    });
+  });
 }