[gesposito]

What broke

Transition searches never look before 1847-01-01, but five zones change their offset on 1844-12-31, when they moved across the date line: Asia/Manila, Pacific/Guam, Pacific/Saipan, Pacific/Palau and Pacific/Kosrae. These are the only transitions before 1847 among the 418 zones of Node 26.12 (tz 2026e). Their offsets are correct, but getTimeZoneTransition() misses them: from 1800, 'next' skips to the 1899 or 1900 change, and from 1846, 'previous' returns null.

Zone	Native next from 1800	temporal-polyfill 1.0.6	Native previous from 1846	temporal-polyfill 1.0.6
Asia/Manila	1844-12-31T15:56:08Z	1899-09-06T04:00:00Z	1844-12-31T15:56:08Z	null
Pacific/Guam	1844-12-31T14:21:00Z	1900-12-31T14:21:00Z	1844-12-31T14:21:00Z	null
Pacific/Saipan	1844-12-31T14:21:00Z	1900-12-31T14:21:00Z	1844-12-31T14:21:00Z	null
Pacific/Palau	1844-12-31T15:02:04Z	1900-12-31T15:02:04Z	1844-12-31T15:02:04Z	null
Pacific/Kosrae	1844-12-31T13:08:04Z	1900-12-31T13:08:04Z	1844-12-31T13:08:04Z	null

import { Temporal } from 'temporal-polyfill/implementation'

Temporal.Instant.from('1800-01-01T00:00Z')
  .toZonedDateTimeISO('Asia/Manila')
  .getTimeZoneTransition('next')
  .toString()
// '1899-09-06T12:00:00+08:00[Asia/Manila]', expected '1845-01-01T00:00:00+08:04[Asia/Manila]'

Why

minPossibleTransitionSec (1847-01-01, the bound proposal-temporal's polyfill uses as BEFORE_FIRST_DST) is where forward searches from the far past start, and below which backward searches give up.

The fix

Lower minPossibleTransitionSec to 1844-01-01. Only searches that start before 1847 are affected.

Tests

Added to polyfill/src/internal/timeZoneImpl.test.ts: Manila from 1800 forwards and from 1846 backwards, and Guam from 1800. Manila's transition instant depends on the host's time zone data: Node 22.14 (tz 2024b) gives Manila's local mean time as −15:56, newer data as −15:56:08, so the test accepts 1844-12-31T15:56:00Z through 15:56:08Z and checks that both directions agree.

Verification

With pnpm 10 and the repo's use-node-version (Node 22.14.0), after git submodule update --init --recursive:

• pnpm run build, pnpm run tsc:all and pnpm run lint: clean
• pnpm run vitest: 939 passed, 7 skipped; the new test also passes with VITEST_NODE_VERSION=26.12.0. On 26.12, two diffUnzoned parity tests fail on main as well.
• pnpm run test262: full 6843 passed, 46 expected failures; basic 5324 passed, 1565 expected failures (the same as main)
• The built dist/implementation.js matches native Temporal for every row of the table above.