# Time Zone-Aware Scheduling

## Objective

Schedule things for the right local time when everyone is in a different zone.

Scheduling that stores an unambiguous instant alongside the intended zone, and survives daylight-saving transitions.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Store both the resolved instant and the IANA zone identifier the user chose. The instant alone loses the intent; the local time alone is ambiguous.
2. Show the user the zone their schedule is in, and show viewers in other zones the same moment converted to theirs with the zone named.
3. Decide per schedule type whether it is a fixed instant or a recurring local wall-clock time, and store that intent explicitly — the two diverge every time the clocks change.
4. Resolve daylight-saving edge cases with a stated rule: a nonexistent local time and a repeated local time both need a defined answer, not whatever the library defaults to.
5. Do NOT store a UTC offset in place of a zone name. Offsets go stale the moment the rules change, and governments change them.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- A local time that does not exist during a spring-forward gap must be shifted by a documented rule and shown to the user.
- A repeated local time in an autumn-back overlap must pick one occurrence deliberately.
- Existing schedules must not shift when a country changes its time-zone rules — historical instants stay fixed.
- A recurring 9am local schedule must remain 9am local across a DST change, which means its UTC time moves.
- The browser-detected zone is a default, not a decision — let the user change it.
- Sends near a transition must not fire twice or be skipped.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Every schedule stores an instant plus an IANA zone identifier.
- [ ] Fixed-instant and recurring-local schedules are distinguishable in the data.
- [ ] Times are displayed in the viewer's zone with the zone named.
- [ ] Nonexistent and ambiguous local times resolve by a documented rule.
- [ ] A DST transition does not duplicate or skip a scheduled run.
- [ ] Time-zone rule updates do not move existing scheduled instants.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.