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Fix POSIX TZ week-of-month calculation near DST transitions #760

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Fix POSIX TZ week-of-month calculation near DST transitions #760

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fe906e0
Fix POSIX TZ week-of-month calculation near DST transitions
abhinav-phi Oct 31, 2026
0e1f8d1
Merge remote-tracking branch 'origin/main' into fix-posix-dst-week
abhinav-phi Nov 1, 2026
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2 changes: 1 addition & 1 deletion crates/provider/src/common.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -250,7 +250,7 @@ impl MwdForTime {
#[cfg(any(feature = "tzif", feature = "experimental_tzif"))]
pub(crate) fn from_seconds(seconds: i64) -> Self {
let (year, month, day_of_month) = utils::ymd_from_epoch_milliseconds(seconds * 1_000);
let week_of_month = day_of_month / 7 + 1;
let week_of_month = (day_of_month - 1) / 7 + 1;
let day_of_week = utils::epoch_seconds_to_day_of_week(seconds);
let mut mwd = Mwd::from_u8(month, week_of_month, day_of_week);
let days_in_month = utils::iso_days_in_month(year, month);
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21 changes: 21 additions & 0 deletions crates/temporal/src/builtins/compiled/plain_date_time.rs
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Expand Up @@ -44,4 +44,25 @@ mod tests {
// Should disambiguate to 2020-03-08T01:30:00-08:00[America/Los_Angeles]
assert_eq!(zdt.hour(), 3);
}

#[cfg(feature = "tzdb")]
#[test]
fn new_york_standard_time_before_dst_start() {
use crate::{options::Disambiguation, tzdb::CompiledTzdbProvider, PlainDateTime, TimeZone};
let provider = &CompiledTzdbProvider::default();

@Manishearth Manishearth Oct 31, 2026

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Please use the test_all_providers macro

temporal/crates/temporal/src/builtins/core/zoned_date_time/tests.rs

Line 18 in 6196a70

macro_rules! test_all_providers {

If necessary, move it into a test_utils.rs file.


// DST starts on 2026-03-08, so 2026-03-07 is still standard time.
let pdt = PlainDateTime::try_new_iso(2026, 3, 7, 12, 0, 0, 0, 0, 0).unwrap();
let zdt = pdt
.to_zoned_date_time_with_provider(
TimeZone::try_from_identifier_str_with_provider("America/New_York", provider)
.unwrap(),
Disambiguation::Compatible,
provider,
)
.unwrap();

assert_eq!(zdt.hour(), 12);
assert_eq!(zdt.offset(), "-05:00");
}
}