Fix edge case for disambiguating ZonedDateTimes on DSTs skipping midnight (#156)

Fixes part of the test
[intl402/Temporal/ZonedDateTime/from/dst-skipped-cross-midnight.js](https://github.com/tc39/test262/blob/d9b10790bc4bb5b3e1aa895f11cbd2d31a5ec743/test/intl402/Temporal/ZonedDateTime/from/dst-skipped-cross-midnight.js).
This doesn't fix correctly resolving the start of the day for days that
don't start at 00:00, since that'll require more advanced logic.

Author: jedel1043

src/components/timezone.rs

@@ -7,6 +7,8 @@ use core::{iter::Peekable, str::Chars};
 
 use num_traits::ToPrimitive;
 
+use crate::components::duration::DateDuration;
+use crate::Calendar;
 use crate::{
     components::{duration::normalized::NormalizedTimeDuration, EpochNanoseconds, Instant},
     iso::{IsoDate, IsoDateTime, IsoTime},
@@ -23,6 +25,8 @@ use std::sync::{LazyLock, Mutex};
 pub static TZ_PROVIDER: LazyLock<Mutex<FsTzdbProvider>> =
     LazyLock::new(|| Mutex::new(FsTzdbProvider::default()));
 
+const NS_IN_HOUR: i128 = 60 * 60 * 1000 * 1000 * 1000;
+
 // NOTE: It may be a good idea to eventually move this into it's
 // own individual crate rather than having it tied directly into `temporal_rs`
 pub trait TzProvider {
@@ -31,7 +35,7 @@ pub trait TzProvider {
     fn get_named_tz_epoch_nanoseconds(
         &self,
         identifier: &str,
-        iso_datetime: IsoDateTime,
+        local_datetime: IsoDateTime,
     ) -> TemporalResult<Vec<EpochNanoseconds>>;
 
     fn get_named_tz_offset_nanoseconds(
@@ -258,13 +262,22 @@ impl TimeZone {
         //    which CompareISODateTime(before, isoDateTime) = -1 and !
         //    GetPossibleEpochNanoseconds(timeZone, before) is not
         //    empty.
-        let mut before = iso;
-        before.time.hour -= 3;
+        let before = iso.add_date_duration(
+            Calendar::default(),
+            &DateDuration::default(),
+            NormalizedTimeDuration(-3 * NS_IN_HOUR),
+            None,
+        )?;
+
         // 7. Let after be the earliest possible ISO Date-Time Record
         //    for which CompareISODateTime(after, isoDateTime) = 1 and !
         //    GetPossibleEpochNanoseconds(timeZone, after) is not empty.
-        let mut after = iso;
-        after.time.hour += 3;
+        let after = iso.add_date_duration(
+            Calendar::default(),
+            &DateDuration::default(),
+            NormalizedTimeDuration(3 * NS_IN_HOUR),
+            None,
+        )?;
 
         // 8. Let beforePossible be !
         //    GetPossibleEpochNanoseconds(timeZone, before).

src/components/zoneddatetime.rs

@@ -29,6 +29,7 @@ use crate::{
 use crate::components::timezone::TZ_PROVIDER;
 
 /// A struct representing a partial `ZonedDateTime`.
+#[derive(Debug, Default, Clone, PartialEq)]
 pub struct PartialZonedDateTime {
     /// The `PartialDate` portion of a `PartialZonedDateTime`
     pub date: PartialDate,
@@ -1117,7 +1118,7 @@ pub(crate) fn interpret_isodatetime_offset(
                     // i. Let roundedCandidateNanoseconds be RoundNumberToIncrement(candidateOffset, 60 × 10**9, half-expand).
                     let rounded_candidate = IncrementRounder::from_potentially_negative_parts(
                         candidate_offset,
-                        unsafe { NonZeroU128::new_unchecked(60_000_000_000) },
+                        const { NonZeroU128::new(60_000_000_000).expect("cannot be zero") },
                     )?
                     .round(TemporalRoundingMode::HalfExpand);
                     // ii. If roundedCandidateNanoseconds = offsetNanoseconds, then
@@ -1173,8 +1174,7 @@ pub(crate) fn nanoseconds_to_formattable_offset_minutes(
     Ok((sign, hour as u8, minute as u8))
 }
 
-#[cfg(feature = "tzdb")]
-#[cfg(test)]
+#[cfg(all(test, feature = "tzdb"))]
 mod tests {
     use crate::{
         options::{Disambiguation, OffsetDisambiguation},
@@ -1344,4 +1344,19 @@ mod tests {
         );
         assert!(result.is_ok());
     }
+
+    #[test]
+    // https://github.com/tc39/test262/blob/d9b10790bc4bb5b3e1aa895f11cbd2d31a5ec743/test/intl402/Temporal/ZonedDateTime/from/dst-skipped-cross-midnight.js
+    fn dst_skipped_cross_midnight() {
+        let provider = &FsTzdbProvider::default();
+        let midnight_disambiguated = ZonedDateTime::from_str_with_provider(
+            "1919-03-31T00[America/Toronto]",
+            Disambiguation::Compatible,
+            OffsetDisambiguation::Reject,
+            provider,
+        )
+        .unwrap();
+
+        assert_eq!(midnight_disambiguated.epoch_milliseconds(), -1601751600000);
+    }
 }

src/tzdb.rs

@@ -92,7 +92,7 @@ impl From<LocalTimeTypeRecord> for LocalTimeRecord {
 
 // TODO: Workshop record name?
 /// The `LocalTimeRecord` result represents the result of searching for a
-/// a for a time zone transition without the offset seconds applied to the
+/// time zone transition without the offset seconds applied to the
 /// epoch seconds.
 ///
 /// As a result of the search, it is possible for the resulting search to be either
@@ -295,7 +295,8 @@ impl Tzif {
         match offset_range.contains(&current_diff.0) {
             true if next_record.is_dst => Ok(LocalTimeRecordResult::Empty),
             true => Ok((next_record, initial_record).into()),
-            false => Ok(initial_record.into()),
+            false if current_diff <= initial_record.utoff => Ok(initial_record.into()),
+            false => Ok(next_record.into()),
         }
     }
 }
@@ -557,14 +558,14 @@ impl TzProvider for FsTzdbProvider {
             LocalTimeRecordResult::Empty => Vec::default(),
             LocalTimeRecordResult::Single(r) => {
                 let epoch_ns =
-                    EpochNanoseconds::try_from(epoch_nanos.0 + seconds_to_nanoseconds(r.offset))?;
+                    EpochNanoseconds::try_from(epoch_nanos.0 - seconds_to_nanoseconds(r.offset))?;
                 vec![epoch_ns]
             }
             LocalTimeRecordResult::Ambiguous { std, dst } => {
                 let std_epoch_ns =
-                    EpochNanoseconds::try_from(epoch_nanos.0 + seconds_to_nanoseconds(std.offset))?;
+                    EpochNanoseconds::try_from(epoch_nanos.0 - seconds_to_nanoseconds(std.offset))?;
                 let dst_epoch_ns =
-                    EpochNanoseconds::try_from(epoch_nanos.0 + seconds_to_nanoseconds(dst.offset))?;
+                    EpochNanoseconds::try_from(epoch_nanos.0 - seconds_to_nanoseconds(dst.offset))?;
                 vec![std_epoch_ns, dst_epoch_ns]
             }
         };