# Time Zone Converter

## Objective

Show a time in both zones at once so nobody joins an hour late.

A paired display of any scheduled time in the viewer's zone and the other party's zone.

## 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. Find every place the app shows a time that two people in different places need to agree on: booking confirmations, invitations, live sessions, deadlines. Show both zones there rather than one canonical time.
2. Detect the viewer's zone from their device on first visit, but store an explicit override on the account and let them change it from settings. Detection is a starting guess, not an answer.
3. Label each zone by city and current offset, not by an abbreviation. Several abbreviations mean two different things depending on the country, and the reader cannot tell which one you meant.
4. If the app already has Time Zone-Aware Scheduling, take its stored zone data and conversion helpers as the source of truth and add only the paired display. Do not introduce a second, differently behaved notion of what zone a user is in.
5. Do not convert by adding a fixed hour offset stored at write time. Offsets change with daylight saving, so a meeting booked in one season silently drifts by an hour in the next.

## 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

- The viewer's zone is detected but must always be overridable, and the override must persist across sessions and devices rather than being re-guessed on each visit.
- Zones are named by city and offset. An abbreviation is ambiguous across countries and leaves the reader unsure which of two times was meant.
- When the two sides fall on different calendar days, say so explicitly next to the time. A reader who sees only the hour will assume it is the same day and be wrong.
- Some zones sit at thirty- or forty-five-minute offsets. Any arithmetic or formatting that assumes whole hours will display those times incorrectly.
- On clock-change days some local times occur twice and some do not occur at all. Warn on both, and resolve the ambiguity explicitly rather than picking one silently.
- A device with a badly wrong clock or an unrecognised zone must fall back to the stored account zone rather than rendering nothing.
- Historical times must be converted using the rules that applied on that date, not today's rules, or past records will shift as governments change their zones.
- The two times must be readable side by side on a narrow screen without one wrapping away from its label.

## 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 shared time displays in both the viewer's zone and the other party's zone.
- [ ] The viewer's zone is detected on first use and can be overridden persistently.
- [ ] Zones are labelled by city and current offset, never by a bare abbreviation.
- [ ] A day difference between the two zones is stated explicitly.
- [ ] Non-whole-hour offsets and daylight-saving transitions render correctly, including ambiguous and non-existent local times.
- [ ] Conversions are computed from the stored instant and zone identifier, not from a cached numeric offset.
- [ ] 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.