website refactor

docs/architecture/website/DISPLAY_OBJECTS.md

@@ -11,6 +11,13 @@ In this codebase, a Display is a **Frontend Value Object**:
 - deterministic
 - side-effect free
 
+### Distinction from Domain Value Objects
+
+While both are "Value Objects", they serve different layers:
+
+1. **Domain Value Objects (Core):** Encapsulate business truth and invariants (e.g., `Money`, `EmailAddress`). They are pure and never contain formatting logic.
+2. **Display Objects (Website):** Encapsulate presentation truth and formatting (e.g., `PriceDisplay`, `DateDisplay`). They are used to transform raw data or Domain Value Objects into user-ready strings.
+
 It answers the question:
 
 > “How should this specific piece of information be shown?”
@@ -65,6 +72,8 @@ A Display MUST NOT:
 - influence system behavior
 - be sent back to the server
 - depend on backend or infrastructure concerns
+- **depend on environment-specific APIs** (e.g., `window`, `document`, `navigator`)
+- **be serialized** (they are classes; only their primitive outputs are stored in `ViewData`)
 
 In this repository, a Display MUST NOT:
 
@@ -73,6 +82,25 @@ In this repository, a Display MUST NOT:
 
 Reason: these are runtime-locale/timezone dependent and cause SSR/hydration mismatches.
 
+### Handling Client-Only Formatting
+
+If a formatting requirement **strictly requires** client-only APIs (e.g., browser-native relative time or local timezone detection):
+
+1. It MUST NOT live in a `Display Object`.
+2. It SHOULD live in a **View Model** (which is client-only).
+3. The Template should handle the transition from server-provided `ViewData` to client-updated `ViewData`.
+
+### Best Practices for Time/Date Formatting
+
+To avoid hydration mismatches while still providing good SEO and UX:
+
+1. **Use UTC methods for Determinism:** In `Display Objects`, prefer `getUTCDate()`, `getUTCMonth()`, etc., over their local counterparts. This ensures the server and client produce the exact same string regardless of their local timezones.
+2. **Hardcoded Arrays:** Use hardcoded arrays for month/day names instead of `Intl` to ensure consistency across environments.
+3. **Pass "Now" as an Argument:** For relative time (e.g., "time ago"), pass the reference "now" timestamp as an argument to the `Display Object` instead of calling `Date.now()` inside it.
+4. **The "Upgrade" Pattern:**
+   - **Server:** `ViewData Builder` uses a `Display Object` to produce a deterministic UTC-based string (e.g., "2024-01-18 15:00 UTC").
+   - **Client:** `View Model` uses client APIs (`Intl`, `toLocale*`) to produce a localized string (e.g., "3:00 PM") and updates the `ViewData`.
+
 ## Localization rule (strict)
 
 Localization MUST NOT depend on runtime locale APIs.
@@ -105,16 +133,17 @@ Placement rule (strict):
 
 ---
 
-## Relationship to View Models
+## Relationship to View Models and ViewData Builders
 
-- View Models MAY use Displays
-- Displays MUST NOT depend on View Models
-- Displays represent **parts**
-- View Models represent **screens**
+Displays are the **shared source of truth** for formatting logic across the website:
+
+- **ViewData Builders (Server):** Use Displays to produce deterministic, formatted strings for SEO and initial SSR.
+- **View Models (Client):** Use Displays to produce formatted strings for interactive UI and client-specific context.
 
 Additional strict rules:
 
 - View Models SHOULD compose Displays.
+- ViewData Builders SHOULD use Displays for all formatting.
 - Displays MUST NOT be serialized or passed across boundaries.
   - They must not appear in server-to-client DTOs.
   - Templates should receive primitive display outputs, not Display instances.
@@ -138,9 +167,17 @@ Additionally:
 
 ## Summary
 
-- Displays encapsulate **how something looks**
-- View Models encapsulate **what a screen needs**
-- Both are presentation concerns
-- Neither contains business truth
+- Displays encapsulate **how something looks** (the single source of truth for formatting logic).
+- View Models encapsulate **what a screen needs** (including client-specific "last mile" formatting).
+- Both are presentation concerns.
+- Neither contains business truth.
 
-In one sentence: Displays are **Value Objects for UI display**, not utility functions.
+In one sentence: **Displays are the shared source of truth for deterministic formatting logic, used by both the server and the client.**
+
+---
+
+## Final Rule: Where does formatting go?
+
+1. **Is it deterministic?** (e.g., currency symbols, fixed date formats, labels) → **Display Object**.
+2. **Is it client-only?** (e.g., `Intl.*`, `toLocale*`, browser timezone) → **View Model**.
+3. **Is it for SEO?** → **ViewData Builder** (using a Display Object).