[ptomato]

This rebase PR covers changes from December 2025 and January 2026. A large part of it consists of deleting code that was determined to be unreachable.

LLMs or human reviewers can look at the SHAs in each commit (e.g. UPSTREAM_COMMIT=bc64d40d8a42b81a3d1cb21a3bc291da9d6d0b8d) to see the corresponding commit in https://github.com/tc39/proposal-temporal.

[Copilot]

Copilot review overview

🔵 Needs a closer look

It is a large, wide-ranging rebase touching core date arithmetic, string parsing, calendar/era logic, and Intl formatting, including a sign-convention flip and many guard-to-assert conversions that warrant final human review.

Review effort: Balanced
Findings: 1

Open (1)

• Fix duplicated article in error message · New

What changed in this PR

This is the fourth in a series of "rebase" PRs that port upstream changes from tc39/proposal-temporal (Dec 2025 – Jan 2026) into this polyfill. The dominant theme is replacing user-facing guard clauses that were determined to be unreachable with internal assert(...) calls, plus several core refactors: a new AddDaysToISODate helper replacing BalanceISODate, a rewrite of the ISO-string parsing regexes to use named capture groups, calendar era handling switching from hasYearZero to a numeric startingYear, and new dateStyle/timeStyle handling in the Intl.DateTimeFormat wrapper.

Changes:

• Convert many unreachable throw guards into assert(...) across ecmascript.ts and calendar.ts; drop redundant === undefined ? 0 param defaults in constructors.
• Rewrite lib/regex.ts parsing regexes to use named capture groups (relies on ES2025 duplicate named groups) and update ParseISODateTime/ParseTemporalTimeString/offset parsing to read match.groups.
• Refactor calendar difference logic (untilCalendar, ISODateSurpasses→CompareSurpasses, NonIsoCalendar.dateUntil week/day fast path), flip the sign convention in DifferenceZonedDateTime, and update Intl dateStyle/timeStyle slot handling; update test expected-failure lists.

File	Description
lib/​ecmascript.ts	Guard→assert conversions, AddDaysToISODate adoption, named-group parsing, DifferenceZonedDateTime sign flip, equal-date short-circuit in CalendarDateUntil.
lib/​calendar.ts	untilCalendar/ISODateSurpasses rewrite, startingYear era model, clampISODate, non-ISO week/day diff fast path, guard→assert conversions.
lib/​regex.ts	Regexes rebuilt with named capture groups for date/time/offset/duration parsing.
lib/​intl.ts	CreateDateTimeFormat export, dateStyle/timeStyle slot handling with nullable formatters, formatRange type check simplification.
lib/​slots.ts	DATE/YM/MD/TIME_FMT slots widened to `FormatterOrAmender
lib/​plain*.ts, duration.ts, zoneddatetime.ts	Dropped redundant undefined param defaults; switched toLocaleString to CreateDateTimeFormat; direction guard→assert.
lib/​math.ts	Removed dead `
index.d.ts	Removed Temporal.ZonedDateTime from Intl.Formattable.
test/​*.txt, runtest262.mjs, test/​validStrings.mjs	Updated expected-failure lists, removed cldr48 handling, year ?? 1972 fallback.

I verified the most correctness-sensitive parts: the DifferenceZonedDateTime sign flip is internally consistent across all its sign uses; the next.day = calendarOne.day in-place mutation and the assertion-based refactors match the upstream reference implementation; the reworked regexes construct and parse correctly on Node 24; the era startingYear migration leaves no stray hasYearZero; and impl.dateUntil is only reached via CalendarDateUntil, so the new equal-date asserts are safe.

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[ptomato]

Pre-existing in the spec polyfill, will fix it there and pull it in here in the next rebase.

[ptomato]

I'll have to do a bit of refactoring to remove the regex duplicate named capture groups. Will push a new version shortly.

[justingrant]

Looks great, only a few minor comments/questions.