# Date Comparison

## Objective

Show how this period compares with the last one.

Period-over-period comparison on analytics figures, with aligned periods, honest handling of incomplete ranges, and sane percentage changes.

## 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. Offer previous period and same period last year as comparison options, and allow a custom comparison range.
2. Make the comparison period exactly the same length as the selected period, and state the compared dates in the UI. An unexplained percentage is not information.
3. When the current period is still running, compare like for like — either compare elapsed portions only, or label the figure as partial. Do not compare four days against thirty and call it a decline.
4. Do all bucketing and boundary arithmetic in the reporting time zone, and show which time zone that is.
5. Show direction with more than colour: an arrow or a sign, so the change is readable to colourblind users and in monochrome print.
6. Do NOT build a new range-selection control — the picker is owned by Date Range Picker; read the selected range from it and extend that feature if it needs a comparison mode.

## 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 zero baseline has no percentage change. Show 'no prior data' or the absolute figure, never infinity or a 100% that means nothing.
- A tiny baseline produces absurd percentages — two to six is +200%. Suppress or annotate changes below a minimum sample.
- Month-over-month comparison across months of different lengths must either align by day count or say it is aligning by calendar month.
- Leap days and daylight-saving transitions make some periods 23 or 25 hours long; bucket boundaries must not drift.
- Same period last year should align by weekday for weekly-seasonal metrics, or the comparison is noise.
- Rate metrics do not subtract like counts — a change in conversion rate is percentage points, not percent.
- Late-arriving data will change yesterday's number after you have already shown a comparison; either wait for the data to settle or label recent figures as provisional.

## 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 comparison states both date ranges explicitly.
- [ ] Comparison periods are equal in length to the selected period.
- [ ] Incomplete current periods are either elapsed-matched or clearly labelled as partial.
- [ ] All boundaries are computed in a stated reporting time zone.
- [ ] Zero and near-zero baselines never produce a misleading percentage.
- [ ] Direction is conveyed by more than colour alone.
- [ ] Range selection continues to be handled by Date Range Picker.
- [ ] 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.