Regional time-zone date conversion across Svelte 5 runes and SvelteKit 2 SSR boundary

Goal

Render a date in a specific IANA time zone consistently during server-side rendering and after client-side hydration.

Constraints

• Svelte 5 replaces reactive assignments with $state/$derived runes, changing how Date objects are tracked for reactivity.
• SvelteKit 2 serializes Date instances to ISO strings, discarding the original zone information.
• The framework provides no built-in timezone API; developers must use Intl.DateTimeFormat or libraries such as Luxon or date-fns-tz.
• Mutating a shared $state Date can cause stale UI across components.

Open questions

1. Should date normalization occur on the server before serialization or on the client after hydration?
2. How can a $state Date store be kept in sync without mutating shared instances?
3. What pattern avoids hydration mismatches when using Intl.DateTimeFormat with a specific zone?