ZonedDateTime backward shift spec change (#725)

From tc39/proposal-temporal#3312

Spec change (https://ptomato.name/talks/tc39-2026-05/#7)

<img width="915" height="327" alt="image"
src="https://github.com/user-attachments/assets/cb788171-8305-4e6f-89db-004f2cb6864d"
/>

Fixes tc39/test262#5047

Author: Manishearth

src/builtins/core/zoned_date_time.rs

@@ -1301,17 +1301,20 @@ impl ZonedDateTime {
             // c. Let startNs be ? GetStartOfDay(timeZone, dateStart).
             // d. Assert: thisNs ≥ startNs.
             // e. Let endNs be ? GetStartOfDay(timeZone, dateEnd).
-            // f. Assert: thisNs < endNs.
+            // f. [Struck out in spec] Assert: thisNs < endNs.
             let start = self.time_zone.get_start_of_day(&iso_start.date, provider)?;
             let end = self.time_zone.get_start_of_day(&iso_end, provider)?;
-            if !(this_ns.0 >= start.ns.0 && this_ns.0 < end.ns.0) {
+            if this_ns.0 < start.ns.0 {
                 return Err(TemporalError::range().with_enum(ErrorMessage::ZDTOutOfDayBounds));
             }
-            // g. Let dayLengthNs be ℝ(endNs - startNs).
-            // h. Let dayProgressNs be TimeDurationFromEpochNanosecondsDifference(thisNs, startNs).
+            // g. Set thisNs to min(thisNs, endNs - 1).
+            let this_ns_val = this_ns.0.min(end.ns.0 - 1);
+            // h. Let dayLengthNs be ℝ(endNs - startNs).
+            // i. Let dayProgressNs be TimeDurationFromEpochNanosecondsDifference(thisNs, startNs).
             let day_len_ns = TimeDuration::from_nanosecond_difference(end.ns.0, start.ns.0)?;
-            let day_progress_ns = TimeDuration::from_nanosecond_difference(this_ns.0, start.ns.0)?;
-            // i. Let roundedDayNs be ! RoundTimeDurationToIncrement(dayProgressNs, dayLengthNs, roundingMode).
+            let day_progress_ns =
+                TimeDuration::from_nanosecond_difference(this_ns_val, start.ns.0)?;
+            // j. Let roundedDayNs be ! RoundTimeDurationToIncrement(dayProgressNs, dayLengthNs, roundingMode).
             let rounded = if let Some(increment) = NonZeroU128::new(day_len_ns.0.unsigned_abs()) {
                 IncrementRounder::<i128>::from_signed_num(day_progress_ns.0, increment)?
                     .round(resolved.rounding_mode)
@@ -1326,7 +1329,7 @@ impl ZonedDateTime {
                 end.offset
             };
 
-            // j. Let epochNanoseconds be AddTimeDurationToEpochNanoseconds(roundedDayNs, startNs).
+            // k. Let epochNanoseconds be AddTimeDurationToEpochNanoseconds(roundedDayNs, startNs).
             let candidate = start.ns.0 + rounded;
             Instant::try_new(candidate)?;
             // 20. Return ! CreateTemporalZonedDateTime(epochNanoseconds, timeZone, calendar).

src/builtins/core/zoned_date_time/tests.rs

@@ -1278,3 +1278,104 @@ fn hours_in_day_dst_changes() {
         assert_eq!(spring.hours_in_day_with_provider(provider), Ok(23.0));
     })
 }
+
+// Case where midnight occurs twice (e.g., Antarctica/Casey on 2010-03-05).
+// Upstream tests: https://github.com/tc39/test262/pull/5047
+#[test]
+fn test_same_date_starts_twice() {
+    test_all_providers!(provider: {
+        let zdt1 = ZonedDateTime::from_utf8_with_provider(
+            b"2010-03-04T23:10:00+11:00[Antarctica/Casey]",
+            Disambiguation::Compatible,
+            OffsetDisambiguation::Use,
+            provider,
+        );
+        if zdt1.is_err() {
+            std::println!("Antarctica/Casey not supported by provider, skipping test.");
+            return;
+        }
+        let zdt1 = zdt1.unwrap();
+
+        let zdt2 = ZonedDateTime::from_utf8_with_provider(
+            b"2010-03-05T00:45:00+11:00[Antarctica/Casey]",
+            Disambiguation::Compatible,
+            OffsetDisambiguation::Use,
+            provider,
+        ).unwrap();
+
+        let zdt3 = ZonedDateTime::from_utf8_with_provider(
+            b"2010-03-04T23:10:00+08:00[Antarctica/Casey]",
+            Disambiguation::Compatible,
+            OffsetDisambiguation::Use,
+            provider,
+        ).unwrap();
+
+        let zdt4 = ZonedDateTime::from_utf8_with_provider(
+            b"2010-03-05T00:45:00+08:00[Antarctica/Casey]",
+            Disambiguation::Compatible,
+            OffsetDisambiguation::Use,
+            provider,
+        ).unwrap();
+
+        let start_of_march4 = "2010-03-04T00:00:00+11:00[Antarctica/Casey]";
+        let start_of_march5 = "2010-03-05T00:00:00+11:00[Antarctica/Casey]";
+        let start_of_march6 = "2010-03-06T00:00:00+08:00[Antarctica/Casey]";
+
+        // Hours in day
+        assert_eq!(zdt1.hours_in_day_with_provider(provider).unwrap(), 24.0);
+        assert_eq!(zdt2.hours_in_day_with_provider(provider).unwrap(), 27.0);
+        assert_eq!(zdt3.hours_in_day_with_provider(provider).unwrap(), 24.0);
+        assert_eq!(zdt4.hours_in_day_with_provider(provider).unwrap(), 27.0);
+
+        // Start of day
+        assert_eq!(zdt1.start_of_day_with_provider(provider).unwrap().to_string_with_provider(provider).unwrap(), start_of_march4);
+        assert_eq!(zdt2.start_of_day_with_provider(provider).unwrap().to_string_with_provider(provider).unwrap(), start_of_march5);
+        assert_eq!(zdt3.start_of_day_with_provider(provider).unwrap().to_string_with_provider(provider).unwrap(), start_of_march4);
+        assert_eq!(zdt4.start_of_day_with_provider(provider).unwrap().to_string_with_provider(provider).unwrap(), start_of_march5);
+
+        // Rounding down
+        for rounding_mode in [RoundingMode::Floor, RoundingMode::Trunc] {
+            let options = RoundingOptions {
+                smallest_unit: Some(Unit::Day),
+                rounding_mode: Some(rounding_mode),
+                ..Default::default()
+            };
+            assert_eq!(zdt1.round_with_provider(options, provider).unwrap().to_string_with_provider(provider).unwrap(), start_of_march4);
+            assert_eq!(zdt2.round_with_provider(options, provider).unwrap().to_string_with_provider(provider).unwrap(), start_of_march5);
+            assert_eq!(zdt3.round_with_provider(options, provider).unwrap().to_string_with_provider(provider).unwrap(), start_of_march4);
+            assert_eq!(zdt4.round_with_provider(options, provider).unwrap().to_string_with_provider(provider).unwrap(), start_of_march5);
+        }
+
+        // Rounding to nearest
+        for rounding_mode in [
+            RoundingMode::HalfCeil,
+            RoundingMode::HalfEven,
+            RoundingMode::HalfExpand,
+            RoundingMode::HalfFloor,
+            RoundingMode::HalfTrunc,
+        ] {
+            let options = RoundingOptions {
+                smallest_unit: Some(Unit::Day),
+                rounding_mode: Some(rounding_mode),
+                ..Default::default()
+            };
+            assert_eq!(zdt1.round_with_provider(options, provider).unwrap().to_string_with_provider(provider).unwrap(), start_of_march5);
+            assert_eq!(zdt2.round_with_provider(options, provider).unwrap().to_string_with_provider(provider).unwrap(), start_of_march5);
+            assert_eq!(zdt3.round_with_provider(options, provider).unwrap().to_string_with_provider(provider).unwrap(), start_of_march5);
+            assert_eq!(zdt4.round_with_provider(options, provider).unwrap().to_string_with_provider(provider).unwrap(), start_of_march5);
+        }
+
+        // Rounding up
+        for rounding_mode in [RoundingMode::Ceil, RoundingMode::Expand] {
+            let options = RoundingOptions {
+                smallest_unit: Some(Unit::Day),
+                rounding_mode: Some(rounding_mode),
+                ..Default::default()
+            };
+            assert_eq!(zdt1.round_with_provider(options, provider).unwrap().to_string_with_provider(provider).unwrap(), start_of_march5);
+            assert_eq!(zdt2.round_with_provider(options, provider).unwrap().to_string_with_provider(provider).unwrap(), start_of_march6);
+            assert_eq!(zdt3.round_with_provider(options, provider).unwrap().to_string_with_provider(provider).unwrap(), start_of_march5);
+            assert_eq!(zdt4.round_with_provider(options, provider).unwrap().to_string_with_provider(provider).unwrap(), start_of_march6);
+        }
+    })
+}