# 5.x → 6.0.0, entry by entry

**Installable `As of 2026-08-21`** — `npm view @ultimat3/core version` answers `7.0.0`, so 6.0.0 is behind `latest` and every entry below is a step you take on the way to it. Run that command anyway rather than trusting this line; a version written into a page goes stale on the next tag.

Seven breaking entries, and the first is a **runtime** refusal with no compile error in front of it.

### Start here — the one edit

Every single-label timezone name except `UTC` is refused. `isValidTimeZone` answers `false`, `canonicalTimeZone` answers `undefined`, `assertTimeZone` throws `X_TIMEZONE_INVALID` — and every `@ultimat3/time` formatter is downstream of that one call. **43 names change answer**, tabulated once under [the `6.0.0` section of `CHANGELOG.md`](https://github.com/developerz-ai/ultimate/blob/main/CHANGELOG.md#600); that table is the source and is deliberately not copied here.

```diff
- formatDate(at, { locale, zone: 'CET' })
+ formatDate(at, { locale, zone: 'Europe/Paris' })
```

### Which class the name is in decides whether the swap is mechanical

| Class | Names | Replacement |
|---|---|---|
| geographic link — 24 of the 43 | `Japan`, `GB`, `Hongkong`, `NZ`, … | the `Area/Location` spelling: `Asia/Tokyo`, `Europe/London`, `Asia/Hong_Kong`, `Pacific/Auckland`. Textual — identical wall clock, identical offset |
| UTC alias | `UCT`, `Universal`, `Zulu` | `UTC` |
| the `GMT` family | `GMT`, `GMT0`, `GMT+0`, `GMT-0`, `Greenwich` | `Etc/GMT`, which still renders the label `GMT`. **Not `UTC`**, which renders `UTC` — same instant, different text on any surface that prints the zone name |
| abbreviation | `CET`, `EET`, `MET`, `WET`, `EST`, `MST`, `HST`, `EST5EDT`, `CST6CDT`, `MST7MDT`, `PST8PDT` | **none, and that is the defect.** An abbreviation names no jurisdiction and carries no DST rule, so only the author knows which city's clock was meant: `Europe/Paris` for `CET`, `America/New_York` for `EST5EDT`, `America/Phoenix` for `MST` |

`Etc/GMT+2` is unaffected — only a **leading** sign is a bare offset, and that `+` sits inside a real zone name. `US/Eastern` and `Asia/Calcutta` are unaffected too: a deprecated two-label alias is still `Area/Location`.

### Where the names hide, and why no build error finds them

`TimeZone` is `string` in `@ultimat3/time`, so `'CET'` compiles. Nothing fails until the call runs.

| Site | Spelling | At 6.0.0 |
|---|---|---|
| a formatter, or zone arithmetic | `zone:` on `formatDate`, `formatDateTime`, `formatRange`, `zonePartsAt`, … | throws `X_TIMEZONE_INVALID` on the first call |
| a scheduled task | `tz:` on `task()` | refused where the task is declared — `task()` validates through `isValidTimeZone`, so this one is caught at boot |
| `app.config.ts` | `defaultTimeZone` | refused at boot — `defineConfig` validates through core's own statement of the structural rule, so a stale key is `X_CONFIG_INVALID` naming the field, with the swap in its `fix:` |
| a client's `x-timezone` header | any of the 43 | no error — `resolveTimeZone` falls through to the configured default, so a hand-written client sending `CET` silently renders in your default zone. Browsers are unaffected: `Intl.DateTimeFormat().resolvedOptions().timeZone` is always `Area/Location` |

Find every candidate:

```sh
grep -rnE "(zone|tz|defaultTimeZone): *'[^/']+'" --include='*.ts' --include='*.tsx' .
```

Run it from the app root. Every hit is a single-label zone; `'UTC'` is the only one already correct.

### Why it changed

`Intl` answers "can I format this", never "is this an IANA zone", and at ICU 78 the two stopped agreeing: Bun 1.4 resolves `CET`, `EST`, `GMT` and `MST` where ICU 75 threw. A **runtime upgrade alone** therefore reopened the "no date without an explicit IANA zone" rule — silently, and in the direction that fails dangerous, because an abbreviation carries no DST rule. The judgement is now structural instead of delegated: an identifier is `Area/Location`, and `UTC` is the one legal exception. That refuses the single-label `backward` links along with the abbreviations, and is meant to — no structural rule keeps `CET` out while letting `Japan` in, both being one label, and the alternative is a denylist that grows with every tzdata release. [#251](https://github.com/developerz-ai/ultimate/issues/251), and [Timezones and dates](/timezones-and-dates) for the rule it restores.

### Fixed, and neither costs an edit

| Fix | What changes for you |
|---|---|
| island JSX compiles through `babel-preset-solid` | client-side Solid reactivity inside an island works at all. An island containing JSX compiled to `React.createElement` and threw `ReferenceError: React is not defined` on first interaction, with the gate green. Two build-time dependencies join `@ultimat3/cli`; zero bytes reach your client bundle ([#243](https://github.com/developerz-ai/ultimate/issues/243)) |
| `@ultimat3/core` loads in a browser bundle | **core's** three module-scope `AsyncLocalStorage` constructions — the request context, the active span, the impersonation reason — moved onto one lazy seam, so `@ultimat3/ui` no longer throws `TypeError: undefined is not a constructor` at module evaluation ([#244](https://github.com/developerz-ai/ultimate/issues/244)). Six more constructions **outside** core were untouched at 6.0.0 and carry the same defect — `@ultimat3/db`, `@ultimat3/entity`, `@ultimat3/ai`; they are `[Unreleased]`, along with the guard that makes the rule a build error ([#255](https://github.com/developerz-ai/ultimate/issues/255)) |

Rebuild to pick either up.