feat: Add timezone architecture for consistent date/time handling

- Create dateUtils.ts with helpers for UTC conversion and timezone display
- Add TimezoneSerializerMixin to include business_timezone in API responses
- Update GeneralSettings timezone dropdown with IANA identifiers
- Apply timezone mixin to Event, TimeBlock, and field mobile serializers
- Document timezone architecture in CLAUDE.md

All times stored in UTC, converted for display based on business timezone.
If business_timezone is null, uses user's local timezone.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -61,6 +61,122 @@ docker compose -f docker-compose.local.yml exec django python manage.py <command
 | `frontend/src/api/client.ts` | Axios API client |
 | `frontend/src/types.ts` | TypeScript interfaces |
 | `frontend/src/i18n/locales/en.json` | Translations |
+| `frontend/src/utils/dateUtils.ts` | Date formatting utilities |
+
+## Timezone Architecture (CRITICAL)
+
+All date/time handling follows this architecture to ensure consistency across timezones.
+
+### Core Principles
+
+1. **Database**: All times stored in UTC
+2. **API Communication**: Always use UTC (both directions)
+3. **API Responses**: Include `business_timezone` field
+4. **Frontend Display**: Convert UTC based on `business_timezone`
+   - If `business_timezone` is set → display in that timezone
+   - If `business_timezone` is null/blank → display in user's local timezone
+
+### Data Flow
+
+```
+FRONTEND (User in Eastern Time selects "Dec 8, 2:00 PM")
+    ↓
+Convert to UTC: "2024-12-08T19:00:00Z"
+    ↓
+Send to API (always UTC)
+    ↓
+DATABASE (stores UTC)
+    ↓
+API RESPONSE:
+{
+  "start_time": "2024-12-08T19:00:00Z",     // Always UTC
+  "business_timezone": "America/Denver"     // IANA timezone (or null for local)
+}
+    ↓
+FRONTEND CONVERTS:
+  - If business_timezone set:  UTC → Mountain Time → "Dec 8, 12:00 PM MST"
+  - If business_timezone null: UTC → User local   → "Dec 8, 2:00 PM EST"
+```
+
+### Frontend Helper Functions
+
+Located in `frontend/src/utils/dateUtils.ts`:
+
+```typescript
+import {
+  toUTC,
+  fromUTC,
+  formatForDisplay,
+  formatDateForDisplay,
+  getDisplayTimezone,
+} from '../utils/dateUtils';
+
+// SENDING TO API - Always convert to UTC
+const apiPayload = {
+  start_time: toUTC(selectedDateTime),  // "2024-12-08T19:00:00Z"
+};
+
+// RECEIVING FROM API - Convert for display
+const displayTime = formatForDisplay(
+  response.start_time,        // UTC from API
+  response.business_timezone  // "America/Denver" or null
+);
+// Result: "Dec 8, 2024 12:00 PM" (in business or local timezone)
+
+// DATE-ONLY fields (time blocks)
+const displayDate = formatDateForDisplay(
+  response.start_date,
+  response.business_timezone
+);
+```
+
+### API Response Requirements
+
+All endpoints returning date/time data MUST include:
+
+```python
+# In serializers or views
+{
+    "start_time": "2024-12-08T19:00:00Z",
+    "business_timezone": business.timezone,  # "America/Denver" or None
+}
+```
+
+### Backend Serializer Mixin
+
+Use `TimezoneSerializerMixin` from `core/mixins.py` to automatically add the timezone field:
+
+```python
+from core.mixins import TimezoneSerializerMixin
+
+class EventSerializer(TimezoneSerializerMixin, serializers.ModelSerializer):
+    class Meta:
+        model = Event
+        fields = [
+            'id', 'start_time', 'end_time',
+            # ... other fields ...
+            'business_timezone',  # Provided by mixin
+        ]
+        read_only_fields = ['business_timezone']
+```
+
+The mixin automatically retrieves the timezone from the tenant context.
+- Returns the IANA timezone string if set (e.g., "America/Denver")
+- Returns `null` if not set (frontend uses user's local timezone)
+
+### Common Mistakes to Avoid
+
+```typescript
+// BAD - Uses browser local time, not UTC
+date.toISOString().split('T')[0]
+
+// BAD - Doesn't respect business timezone setting
+new Date(utcString).toLocaleString()
+
+// GOOD - Use helper functions
+toUTC(date)                                   // For API requests
+formatForDisplay(utcString, businessTimezone) // For displaying
+```
 
 ## Key Django Apps