# Decision Log: capacity-expert-mode

## 2026-02-24 — Timezone & Accessibility Fixes

### Issue
User reported that weekend shading in Expert Mode was misaligned (showing on 5th/6th instead of 4th/5th for April 2026). Additionally, the shading was too subtle for colorblind users, and cells were not prefilled with `O`/`H` for weekends/holidays.

### Root Cause
Browser `new Date('YYYY-MM-DD')` interprets dates in local timezone, causing dates to shift for users west of UTC. A user in UTC-6 sees `2026-04-04` as `2026-04-03T18:00`, which reports as Friday instead of Saturday.

### Decisions Made

| # | Decision | Rationale |
|---|----------|-----------|
| D8 | Use Eastern Time (America/New_York) as canonical timezone for weekend/holiday detection | Ensures consistent business-day alignment; future configurable |
| D9 | Use high-contrast styling for weekends/holidays (solid backgrounds + borders) | Accessibility for colorblind users |
| D10 | Prefill weekends with `O`, holidays with `H` on grid load | Matches user expectations; reduces manual entry |
| D11 | Frontend and backend must sync when seeding default values | Prevents divergent behavior on refresh |

### Implementation Notes
- Frontend: Parse dates as `new Date(dateStr + 'T00:00:00')` to avoid timezone drift
- Backend: Use Carbon with `America/New_York` timezone
- Both sides must implement identical prefill logic

### Future Considerations
- Make timezone configurable per-team or per-user (v2)
- Extract prefill rules to shared configuration

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## Earlier Decisions

See `design.md` sections D1-D7 for original design decisions.