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+# Phase 18: Auto timezone detection and ability to change timezone - Context
+
+**Gathered:** 2026-06-14
+**Status:** Ready for planning
+
+<domain>
+## Phase Boundary
+
+Make the household timezone an **explicit, stored, user-changeable setting** —
+auto-detected from the browser — instead of the implicit `process.env.TZ`
+fallback that all-day reminder computation relies on today.
+
+Today the timezone is invisible: the server uses
+`process.env.TZ ?? Intl.DateTimeFormat().resolvedOptions().timeZone` (which is
+UTC inside Docker) purely for the all-day "9 AM local" reminder computation,
+while the browser uses its own local zone for write serialization and display.
+This phase stores a single household timezone, seeds it from the browser at
+setup, lets an admin change it, and routes the server-side all-day reminder
+computation through it.
+
+**In scope:**
+- Store a single household timezone (new `app_config` key, e.g. `household_timezone`).
+- Auto-detect the browser IANA timezone and seed the stored value during the
+  Phase 12 setup wizard / first run.
+- Admin-facing UI to view and change the household timezone (searchable IANA picker).
+- Route the server-side **all-day reminder** "9 AM local" computation through the
+  stored timezone (overriding the `process.env.TZ` fallback when set).
+
+**Out of scope:**
+- Per-member timezones (single household value only — see D-01).
+- Auto-overwrite of the stored timezone on login/travel/VPN drift (see D-03).
+- Changing display rendering or timed-event serialization, which stay
+  browser-local and already-correct (see D-07).
+
+</domain>
+
+<decisions>
+## Implementation Decisions
+
+### Timezone scope
+- **D-01:** Single **household-wide** timezone, stored in `app_config` (one row,
+  e.g. key `household_timezone`, IANA string value). Chosen over a per-member
+  `users.timezone` column — the household is two people in the same location, and
+  a single value matches the existing implicit single-TZ model. No new column or
+  per-row scheduler logic.
+
+### Detection & drift
+- **D-02:** Auto-detect the browser IANA timezone
+  (`Intl.DateTimeFormat().resolvedOptions().timeZone`) and use it to **seed** the
+  stored value during the Phase 12 setup wizard / first run.
+- **D-03:** After seeding, the timezone changes **only** via the settings UI. Do
+  NOT auto-overwrite the stored value on later login/detection differences
+  (travel, VPN, new device). Silent drift would unexpectedly shift the "fires at
+  9 AM" guarantee. (Optional drift *notice* is allowed but not required — never an
+  automatic write.)
+
+### Where to change it
+- **D-04:** Surface the timezone in the existing **role-gated `/admin` Settings**
+  (Phase 10 `requireAdmin` boundary) and seed it from the **Phase 12 setup
+  wizard**. It is a single household-wide setting, so admin-gating is the correct
+  fit; server-side `requireAdmin` is the real boundary (client `isAdmin` is UX
+  only, per Phase 10 D-03).
+
+### Behavioral boundary
+- **D-05:** The stored timezone becomes the **source of truth for the
+  server-side all-day "9 AM local" reminder computation**
+  (`reminderScheduler.ts`, `outboxWorker.ts`), replacing the bare
+  `process.env.TZ ?? Intl…` lookup at those sites.
+- **D-06:** **Fallback chain when `household_timezone` is unset** (e.g. before the
+  wizard seeds it, or on upgrade of an existing install): fall back to the
+  current behavior — `process.env.TZ ?? Intl.DateTimeFormat().resolvedOptions().timeZone`.
+  Nothing breaks before a value is stored; this is the backward-compat path.
+- **D-07:** Display rendering and timed-event write serialization stay
+  **browser-local** and unchanged. This phase must NOT touch the already-correct
+  browser-local write/display path (`eventDateTime.ts`, `hydrateEvents.ts`) —
+  keep the blast radius to the all-day reminder computation only.
+
+### Claude's Discretion
+- Timezone picker UX: a **searchable IANA dropdown** (recorded default; planner/UI
+  may refine). Validate the value is a real IANA zone before storing.
+- Exact `app_config` key name and the read/cache strategy for the stored value in
+  the scheduler/outbox (e.g. read-per-run vs cached) — planner's call.
+- Whether to show a non-blocking "detected zone differs" notice on login (allowed
+  per D-03, not required).
+
+</decisions>
+
+<canonical_refs>
+## Canonical References
+
+**Downstream agents MUST read these before planning or implementing.**
+
+### Current timezone touchpoints (the code this phase rewires)
+- `apps/api/src/broker/reminderScheduler.ts` §~247 — `serverTz` lookup for the
+  all-day branch; primary site to route through the stored timezone (D-05).
+- `apps/api/src/broker/outboxWorker.ts` §~501, §~607 — `tz` lookups feeding
+  `computeAlertInstantUtc` for all-day alert instants; same rewire (D-05).
+- `apps/api/src/broker/vevent.ts` §~233–315 — `computeAlertInstantUtc`: the
+  pure 9 AM-local computation that *consumes* the tz; contract unchanged, only
+  the tz it's fed changes.
+
+### Storage & settings surfaces
+- `apps/api/src/db/schema.ts` §282 — `appConfig` table (key/value/updatedAt);
+  where the `household_timezone` key lives (D-01).
+- Phase 10 admin surface — `apps/api/src/` `/api/admin/*` routes +
+  `requireAdmin`, and the PWA `/admin` Settings route; where the change-tz UI
+  attaches (D-04). See `.planning/phases/10-admin-role-settings/10-CONTEXT.md`.
+- Phase 12 setup wizard — `.planning/phases/12-*/12-CONTEXT.md` and its
+  `12-UI-SPEC.md`; where detection seeding hooks in (D-02). **Note dependency
+  ordering:** Phase 12 may not be executed yet — planner should confirm and, if
+  needed, make wizard seeding additive/optional so Phase 18 isn't blocked.
+
+### Browser-local path (do NOT modify — boundary guard)
+- `apps/pwa/src/lib/eventDateTime.ts` — browser-local write serialization (D-07).
+- `apps/pwa/src/lib/hydrateEvents.ts` — display hydration (D-07).
+
+</canonical_refs>
+
+<code_context>
+## Existing Code Insights
+
+### Reusable Assets
+- `appConfig` table (`schema.ts:282`) — already exists from Phase 10; store the
+  household timezone as a single key/value row, no migration needed for a new key
+  (only seed/read logic).
+- `requireAdmin` gating + `/admin` Settings route (Phase 10) — reuse directly for
+  the change-tz control; no new auth surface.
+- Setup wizard scaffolding (Phase 12) — reuse for first-run detection + seeding.
+- `Intl.DateTimeFormat().resolvedOptions().timeZone` — already the detection
+  primitive used server-side; reuse client-side for seeding.
+
+### Established Patterns
+- Server is the real authorization boundary; client role flags are UX only
+  (Phase 10 D-03) — apply to any tz-write endpoint.
+- All-day reminder math is isolated in `computeAlertInstantUtc` (pure, tz-arg) —
+  the phase only needs to change *what tz is passed*, not the math.
+- Backward-compat via additive `app_config` keys + fallback defaults (Phase 10 /
+  Phase 11 style) — keep the `process.env.TZ ?? Intl` fallback (D-06).
+
+### Integration Points
+- All-day reminder fire path: `reminderScheduler.ts` and `outboxWorker.ts` both
+  independently read tz — both must route through the same stored-value accessor
+  (single helper to avoid drift between the two sites).
+- Admin Settings (read/write tz) ↔ `app_config` ↔ scheduler/outbox read path.
+
+</code_context>
+
+<specifics>
+## Specific Ideas
+
+- The "9 AM local" all-day reminder guarantee is the load-bearing reason this
+  setting exists — correctness of that fire time is the success measure.
+- Keep the change tightly scoped to the all-day reminder computation; the
+  browser-local timed/display path was deliberately fixed earlier and must not
+  regress.
+
+</specifics>
+
+<deferred>
+## Deferred Ideas
+
+- **Per-member timezones** — only relevant if household members live in different
+  zones; revisit as its own phase if that ever becomes true (would add
+  `users.timezone` + per-row scheduler logic).
+- **Driving display/timed reminders off the stored tz** — broader behavior change
+  deliberately excluded (D-07) to protect the already-correct browser-local path.
+
+</deferred>
+
+---
+
+*Phase: 18-auto-timezone-detection-and-ability-to-change-timezone*
+*Context gathered: 2026-06-14*
diff --git a/.planning/phases/18-auto-timezone-detection-and-ability-to-change-timezone/18-DISCUSSION-LOG.md b/.planning/phases/18-auto-timezone-detection-and-ability-to-change-timezone/18-DISCUSSION-LOG.md
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+# Phase 18: Auto timezone detection and ability to change timezone - Discussion Log
+
+> **Audit trail only.** Do not use as input to planning, research, or execution agents.
+> Decisions are captured in CONTEXT.md — this log preserves the alternatives considered.
+
+**Date:** 2026-06-14
+**Phase:** 18-auto-timezone-detection-and-ability-to-change-timezone
+**Areas discussed:** Timezone scope, Detection & drift, Where to change it, Behavioral boundary
+
+---
+
+## Timezone scope
+
+| Option | Description | Selected |
+|--------|-------------|----------|
+| Single household tz | One value in app_config; matches existing implicit single-TZ model and the 2-person same-household reality | ✓ |
+| Per-member timezone | users.timezone column; per-member 9 AM reminders; needs migration + per-row scheduler logic | |
+
+**User's choice:** Single household tz (app_config)
+**Notes:** Two-person same household — single value is the simplest correct model.
+
+---
+
+## Detection & drift
+
+| Option | Description | Selected |
+|--------|-------------|----------|
+| Seed on setup, manual after | Auto-detect during Phase 12 wizard/first run to seed; thereafter changes only via settings UI, no auto-overwrite | ✓ |
+| Detect every login, prompt on drift | Compare browser tz to stored on each login; prompt to update when different | |
+| Detect every login, silent update | Always overwrite stored tz with the browser's | |
+
+**User's choice:** Seed on setup, manual after
+**Notes:** Avoids travel/VPN silently shifting the "fires at 9 AM" guarantee.
+
+---
+
+## Where to change it
+
+| Option | Description | Selected |
+|--------|-------------|----------|
+| Admin Settings + wizard | Role-gated /admin Settings (Phase 10) + seeded by the Phase 12 setup wizard | ✓ |
+| Any member, general settings | Non-admin settings area; fits per-member scope better | |
+
+**User's choice:** Admin Settings + wizard
+**Notes:** Single household-wide setting behind requireAdmin is the correct fit.
+
+---
+
+## Behavioral boundary
+
+| Option | Description | Selected |
+|--------|-------------|----------|
+| All-day reminder tz only | Stored tz is source of truth for server-side all-day 9 AM computation, overriding process.env.TZ; display + timed serialization stay browser-local | ✓ |
+| Also override display / timed logic | Stored tz also drives display rendering and/or timed reminders | |
+
+**User's choice:** All-day reminder tz only
+**Notes:** Keeps blast radius small; protects the already-correct browser-local write/display path.
+
+---
+
+## Claude's Discretion
+
+- Timezone picker UX (recorded default: searchable IANA dropdown; validate IANA before storing).
+- Exact app_config key name + read/cache strategy for the stored value in scheduler/outbox.
+- Optional non-blocking "detected zone differs" login notice (allowed, not required).
+
+## Deferred Ideas
+
+- Per-member timezones — only relevant if members live in different zones; own phase if ever true.
+- Driving display/timed reminders off the stored tz — deliberately excluded to protect the browser-local path.