add rule date time

AGENTS.md

@@ -412,6 +412,22 @@ Notes:
 
 ---
 
+## Date & Time Formatting Rules
+
+- All date and time rendering must use the shared formatter in `src/lib/date-format.ts`.
+- Never use `Date.prototype.toLocaleString()`, `toLocaleDateString()`, `toLocaleTimeString()`, or `Intl.DateTimeFormat()` directly inside React components.
+- UI date display standard:
+  - Date: `YYYY-MM-DD`
+  - DateTime: `YYYY-MM-DD HH:mm:ss`
+  - Time: `HH:mm:ss`
+- API responses must remain ISO-8601 UTC.
+- UI is responsible for formatting.
+- Timezone is fixed to `Asia/Bangkok`.
+- Calendar system must always use Gregorian (AD), never Buddhist Era (BE).
+- All formatting must be deterministic to ensure identical SSR and Client rendering and prevent React hydration mismatches.
+
+---
+
 ## Authentication Patterns
 
 ### Protected Routes